Added nullable annotations to Avalonia.Animation.

src/Avalonia.Animation/Animatable.cs

@@ -157,7 +157,7 @@ namespace Avalonia.Animation
                             state.Instance?.Dispose();
                             state.Instance = transition.Apply(
                                 this,
-                                Clock ?? AvaloniaLocator.Current.GetService<IGlobalClock>(),
+                                Clock ?? AvaloniaLocator.Current.GetRequiredService<IGlobalClock>(),
                                 oldValue,
                                 newValue);
                             return;
@@ -169,7 +169,7 @@ namespace Avalonia.Animation
             base.OnPropertyChangedCore(change);
         }
 
-        private void TransitionsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void TransitionsCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             if (!_transitionsEnabled)
             {
@@ -179,14 +179,14 @@ namespace Avalonia.Animation
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
-                    AddTransitions(e.NewItems);
+                    AddTransitions(e.NewItems!);
                     break;
                 case NotifyCollectionChangedAction.Remove:
-                    RemoveTransitions(e.OldItems);
+                    RemoveTransitions(e.OldItems!);
                     break;
                 case NotifyCollectionChangedAction.Replace:
-                    RemoveTransitions(e.OldItems);
-                    AddTransitions(e.NewItems);
+                    RemoveTransitions(e.OldItems!);
+                    AddTransitions(e.NewItems!);
                     break;
                 case NotifyCollectionChangedAction.Reset:
                     throw new NotSupportedException("Transitions collection cannot be reset.");
@@ -204,7 +204,7 @@ namespace Avalonia.Animation
 
             for (var i = 0; i < items.Count; ++i)
             {
-                var t = (ITransition)items[i];
+                var t = (ITransition)items[i]!;
 
                 _transitionState.Add(t, new TransitionState
                 {
@@ -222,7 +222,7 @@ namespace Avalonia.Animation
 
             for (var i = 0; i < items.Count; ++i)
             {
-                var t = (ITransition)items[i];
+                var t = (ITransition)items[i]!;
 
                 if (_transitionState.TryGetValue(t, out var state))
                 {

src/Avalonia.Animation/Animation.cs

@@ -203,7 +203,7 @@ namespace Avalonia.Animation
         /// </summary>
         /// <param name="setter">The animation setter.</param>
         /// <returns>The property animator type.</returns>
-        public static Type GetAnimator(IAnimationSetter setter)
+        public static Type? GetAnimator(IAnimationSetter setter)
         {
             if (s_animators.TryGetValue(setter, out var type))
             {
@@ -254,7 +254,7 @@ namespace Avalonia.Animation
             Animators.Insert(0, (condition, typeof(TAnimator)));
         }
 
-        private static Type GetAnimatorType(AvaloniaProperty property)
+        private static Type? GetAnimatorType(AvaloniaProperty property)
         {
             foreach (var (condition, type) in Animators)
             {
@@ -276,6 +276,11 @@ namespace Avalonia.Animation
             {
                 foreach (var setter in keyframe.Setters)
                 {
+                    if (setter.Property is null)
+                    {
+                        throw new InvalidOperationException("No Setter property assigned.");
+                    }
+
                     var handler = Animation.GetAnimator(setter) ?? GetAnimatorType(setter.Property);
 
                     if (handler == null)
@@ -305,7 +310,7 @@ namespace Avalonia.Animation
 
             foreach (var (handlerType, property) in handlerList)
             {
-                var newInstance = (IAnimator)Activator.CreateInstance(handlerType);
+                var newInstance = (IAnimator)Activator.CreateInstance(handlerType)!;
                 newInstance.Property = property;
                 newAnimatorInstances.Add(newInstance);
             }
@@ -321,32 +326,43 @@ namespace Avalonia.Animation
         }
 
         /// <inheritdoc/>
-        public IDisposable Apply(Animatable control, IClock clock, IObservable<bool> match, Action onComplete)
+        public IDisposable Apply(Animatable control, IClock? clock, IObservable<bool> match, Action? onComplete)
         {
             var (animators, subscriptions) = InterpretKeyframes(control);
             if (animators.Count == 1)
             {
-                subscriptions.Add(animators[0].Apply(this, control, clock, match, onComplete));
+                var subscription = animators[0].Apply(this, control, clock, match, onComplete);
+                
+                if (subscription is not null)
+                {
+                    subscriptions.Add(subscription);
+                }
             }
             else
             {
                 var completionTasks = onComplete != null ? new List<Task>() : null;
                 foreach (IAnimator animator in animators)
                 {
-                    Action animatorOnComplete = null;
+                    Action? animatorOnComplete = null;
                     if (onComplete != null)
                     {
-                        var tcs = new TaskCompletionSource<object>();
+                        var tcs = new TaskCompletionSource<object?>();
                         animatorOnComplete = () => tcs.SetResult(null);
-                        completionTasks.Add(tcs.Task);
+                        completionTasks!.Add(tcs.Task);
+                    }
+
+                    var subscription = animator.Apply(this, control, clock, match, animatorOnComplete);
+
+                    if (subscription is not null)
+                    {
+                        subscriptions.Add(subscription);
                     }
-                    subscriptions.Add(animator.Apply(this, control, clock, match, animatorOnComplete));
                 }
 
                 if (onComplete != null)
                 {
-                    Task.WhenAll(completionTasks).ContinueWith(
-                        (_, state) => ((Action)state).Invoke(),
+                    Task.WhenAll(completionTasks!).ContinueWith(
+                        (_, state) => ((Action)state!).Invoke(),
                         onComplete);
                 }
             }
@@ -354,25 +370,25 @@ namespace Avalonia.Animation
         }
 
         /// <inheritdoc/>
-        public Task RunAsync(Animatable control, IClock clock = null)
+        public Task RunAsync(Animatable control, IClock? clock = null)
         {
             return RunAsync(control, clock, default);
         }
 
         /// <inheritdoc/>
-        public Task RunAsync(Animatable control, IClock clock = null, CancellationToken cancellationToken = default)
+        public Task RunAsync(Animatable control, IClock? clock = null, CancellationToken cancellationToken = default)
         {
             if (cancellationToken.IsCancellationRequested)
             {
                 return Task.CompletedTask;
             }
 
-            var run = new TaskCompletionSource<object>();
+            var run = new TaskCompletionSource<object?>();
 
             if (this.IterationCount == IterationCount.Infinite)
                 run.SetException(new InvalidOperationException("Looping animations must not use the Run method."));
 
-            IDisposable subscriptions = null, cancellation = null;
+            IDisposable? subscriptions = null, cancellation = null;
             subscriptions = this.Apply(control, clock, Observable.Return(true), () =>
             {
                 run.TrySetResult(null);

src/Avalonia.Animation/AnimationInstance`1.cs

@@ -31,15 +31,15 @@ namespace Avalonia.Animation
         private TimeSpan _initialDelay;
         private TimeSpan _iterationDelay;
         private TimeSpan _duration;
-        private Easings.Easing _easeFunc;
-        private Action _onCompleteAction;
+        private Easings.Easing? _easeFunc;
+        private Action? _onCompleteAction;
         private Func<double, T, T> _interpolator;
-        private IDisposable _timerSub;
+        private IDisposable? _timerSub;
         private readonly IClock _baseClock;
-        private IClock _clock;
-        private EventHandler<AvaloniaPropertyChangedEventArgs> _propertyChangedDelegate;
+        private IClock? _clock;
+        private EventHandler<AvaloniaPropertyChangedEventArgs>? _propertyChangedDelegate;
 
-        public AnimationInstance(Animation animation, Animatable control, Animator<T> animator, IClock baseClock, Action OnComplete, Func<double, T, T> Interpolator)
+        public AnimationInstance(Animation animation, Animatable control, Animator<T> animator, IClock baseClock, Action? OnComplete, Func<double, T, T> Interpolator)
         {
             _animator = animator;
             _animation = animation;
@@ -47,6 +47,9 @@ namespace Avalonia.Animation
             _onCompleteAction = OnComplete;
             _interpolator = Interpolator;
             _baseClock = baseClock;
+            _lastInterpValue = default!;
+            _firstKFValue = default!;
+            _neutralValue = default!;
             FetchProperties();
         }
 
@@ -82,7 +85,7 @@ namespace Avalonia.Animation
 
             _targetControl.PropertyChanged -= _propertyChangedDelegate;
             _timerSub?.Dispose();
-            _clock.PlayState = PlayState.Stop;
+            _clock!.PlayState = PlayState.Stop;
         }
 
         protected override void Subscribed()
@@ -108,6 +111,8 @@ namespace Avalonia.Animation
 
         private void ApplyFinalFill()
         {
+            if (_animator.Property is null)
+                throw new InvalidOperationException("Animator has no property specified.");
             if (_fillMode == FillMode.Forward || _fillMode == FillMode.Both)
                 _targetControl.SetValue(_animator.Property, _lastInterpValue, BindingPriority.LocalValue);
         }
@@ -130,12 +135,12 @@ namespace Avalonia.Animation
 
         private void DoPlayStates()
         {
-            if (_clock.PlayState == PlayState.Stop || _baseClock.PlayState == PlayState.Stop)
+            if (_clock!.PlayState == PlayState.Stop || _baseClock.PlayState == PlayState.Stop)
                 DoComplete();
 
             if (!_gotFirstKFValue)
             {
-                _firstKFValue = (T)_animator.First().Value;
+                _firstKFValue = (T)_animator.First().Value!;
                 _gotFirstKFValue = true;
             }
         }
@@ -169,7 +174,7 @@ namespace Avalonia.Animation
                 // and snap the last iteration value to exact values.
                 if ((_currentIteration + 1) > _iterationCount)
                 {
-                    var easedTime = _easeFunc.Ease(_playbackReversed ? 0.0 : 1.0);
+                    var easedTime = _easeFunc!.Ease(_playbackReversed ? 0.0 : 1.0);
                     _lastInterpValue = _interpolator(easedTime, _neutralValue);
                     DoComplete();
                 }
@@ -203,7 +208,7 @@ namespace Avalonia.Animation
                         normalizedTime = 1 - normalizedTime;
 
                     // Ease and interpolate
-                    var easedTime = _easeFunc.Ease(normalizedTime);
+                    var easedTime = _easeFunc!.Ease(normalizedTime);
                     _lastInterpValue = _interpolator(easedTime, _neutralValue);
 
                     PublishNext(_lastInterpValue);
@@ -223,14 +228,14 @@ namespace Avalonia.Animation
 
         private void UpdateNeutralValue()
         {
-            var property = _animator.Property;
+            var property = _animator.Property ?? throw new InvalidOperationException("Animator has no property specified.");
             var baseValue = _targetControl.GetBaseValue(property, BindingPriority.LocalValue);
 
             _neutralValue = baseValue != AvaloniaProperty.UnsetValue ?
-                (T)baseValue : (T)_targetControl.GetValue(property);
+                (T)baseValue! : (T)_targetControl.GetValue(property)!;
         }
 
-        private void ControlPropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+        private void ControlPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
         {
             if (e.Property == _animator.Property && e.Priority > BindingPriority.Animation)
             {

src/Avalonia.Animation/AnimatorKeyFrame.cs

@@ -12,22 +12,22 @@ namespace Avalonia.Animation
     /// </summary>
     public class AnimatorKeyFrame : AvaloniaObject
     {
-        public static readonly DirectProperty<AnimatorKeyFrame, object> ValueProperty =
-            AvaloniaProperty.RegisterDirect<AnimatorKeyFrame, object>(nameof(Value), k => k.Value, (k, v) => k.Value = v);
+        public static readonly DirectProperty<AnimatorKeyFrame, object?> ValueProperty =
+            AvaloniaProperty.RegisterDirect<AnimatorKeyFrame, object?>(nameof(Value), k => k.Value, (k, v) => k.Value = v);
 
         public AnimatorKeyFrame()
         {
 
         }
 
-        public AnimatorKeyFrame(Type animatorType, Cue cue)
+        public AnimatorKeyFrame(Type? animatorType, Cue cue)
         {
             AnimatorType = animatorType;
             Cue = cue;
             KeySpline = null;
         }
 
-        public AnimatorKeyFrame(Type animatorType, Cue cue, KeySpline keySpline)
+        public AnimatorKeyFrame(Type? animatorType, Cue cue, KeySpline? keySpline)
         {
             AnimatorType = animatorType;
             Cue = cue;
@@ -35,14 +35,14 @@ namespace Avalonia.Animation
         }
 
         internal bool isNeutral;
-        public Type AnimatorType { get; }
+        public Type? AnimatorType { get; }
         public Cue Cue { get; }
-        public KeySpline KeySpline { get; }
-        public AvaloniaProperty Property { get; private set; }
+        public KeySpline? KeySpline { get; }
+        public AvaloniaProperty? Property { get; private set; }
 
-        private object _value;
+        private object? _value;
 
-        public object Value
+        public object? Value
         {
             get => _value;
             set => SetAndRaise(ValueProperty, ref _value, value);
@@ -80,7 +80,7 @@ namespace Avalonia.Animation
                 throw new InvalidCastException($"KeyFrame value doesnt match property type.");
             }
 
-            return (T)typeConv.ConvertTo(Value, typeof(T));
+            return (T)typeConv.ConvertTo(Value, typeof(T))!;
         }
     }
 }

src/Avalonia.Animation/Animators/Animator`1.cs

@@ -24,7 +24,7 @@ namespace Avalonia.Animation.Animators
         /// <summary>
         /// Gets or sets the target property for the keyframe.
         /// </summary>
-        public AvaloniaProperty Property { get; set; }
+        public AvaloniaProperty? Property { get; set; }
 
         public Animator()
         {
@@ -33,7 +33,7 @@ namespace Avalonia.Animation.Animators
         }
 
         /// <inheritdoc/>
-        public virtual IDisposable Apply(Animation animation, Animatable control, IClock clock, IObservable<bool> match, Action onComplete)
+        public virtual IDisposable? Apply(Animation animation, Animatable control, IClock? clock, IObservable<bool> match, Action? onComplete)
         {
             if (!_isVerifiedAndConverted)
                 VerifyConvertKeyFrames();
@@ -106,13 +106,16 @@ namespace Avalonia.Animation.Animators
 
         public virtual IDisposable BindAnimation(Animatable control, IObservable<T> instance)
         {
+            if (Property is null)
+                throw new InvalidOperationException("Animator has no property specified.");
+
             return control.Bind((AvaloniaProperty<T>)Property, instance, BindingPriority.Animation);
         }
 
         /// <summary>
         /// Runs the KeyFrames Animation.
         /// </summary>
-        internal IDisposable Run(Animation animation, Animatable control, IClock clock, Action onComplete)
+        internal IDisposable Run(Animation animation, Animatable control, IClock? clock, Action? onComplete)
         {
             var instance = new AnimationInstance<T>(
                 animation,

src/Avalonia.Animation/Avalonia.Animation.csproj

@@ -2,9 +2,13 @@
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
   </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="..\Avalonia.Base\Metadata\NullableAttributes.cs" Link="NullableAttributes.cs" />
+  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Avalonia.Base\Avalonia.Base.csproj" />
   </ItemGroup>
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\ApiDiff.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
 </Project>

src/Avalonia.Animation/Clock.cs

@@ -4,7 +4,7 @@ namespace Avalonia.Animation
 {
     public class Clock : ClockBase
     {
-        public static IClock GlobalClock => AvaloniaLocator.Current.GetService<IGlobalClock>();
+        public static IClock GlobalClock => AvaloniaLocator.Current.GetRequiredService<IGlobalClock>();
 
         private readonly IDisposable _parentSubscription;
 

src/Avalonia.Animation/Cue.cs

@@ -30,7 +30,7 @@ namespace Avalonia.Animation
         /// <summary>
         /// Parses a string to a <see cref="Cue"/> object.
         /// </summary>
-        public static Cue Parse(string value, CultureInfo culture)
+        public static Cue Parse(string value, CultureInfo? culture)
         {
             string v = value;
 
@@ -72,14 +72,14 @@ namespace Avalonia.Animation
 
     public class CueTypeConverter : TypeConverter
     {
-        public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
+        public override bool CanConvertFrom(ITypeDescriptorContext? context, Type sourceType)
         {
             return sourceType == typeof(string);
         }
 
-        public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
+        public override object ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object value)
         {
             return Cue.Parse((string)value, culture);
         }
     }
-}
+}

src/Avalonia.Animation/DisposeAnimationInstanceSubject.cs

@@ -8,15 +8,15 @@ namespace Avalonia.Animation
     /// </summary>
     internal class DisposeAnimationInstanceSubject<T> : IObserver<bool>, IDisposable
     {
-        private IDisposable _lastInstance;
+        private IDisposable? _lastInstance;
         private bool _lastMatch;
         private Animator<T> _animator;
         private Animation _animation;
         private Animatable _control;
-        private Action _onComplete;
-        private IClock _clock;
+        private Action? _onComplete;
+        private IClock? _clock;
 
-        public DisposeAnimationInstanceSubject(Animator<T> animator, Animation animation, Animatable control, IClock clock, Action onComplete)
+        public DisposeAnimationInstanceSubject(Animator<T> animator, Animation animation, Animatable control, IClock? clock, Action? onComplete)
         {
             this._animator = animator;
             this._animation = animation;

src/Avalonia.Animation/Easing/Easing.cs

@@ -15,7 +15,7 @@ namespace Avalonia.Animation.Easings
         /// <inheritdoc/>
         public abstract double Ease(double progress);
 
-        static Dictionary<string, Type> _easingTypes;
+        static Dictionary<string, Type>? _easingTypes;
 
         static readonly Type s_thisType = typeof(Easing);
 
@@ -48,7 +48,7 @@ namespace Avalonia.Animation.Easings
             if (_easingTypes.ContainsKey(e))
             {
                 var type = _easingTypes[e];
-                return (Easing)Activator.CreateInstance(type);
+                return (Easing)Activator.CreateInstance(type)!;
             }
             else
             {

src/Avalonia.Animation/Easing/EasingTypeConverter.cs

@@ -6,12 +6,12 @@ namespace Avalonia.Animation.Easings
 {
     public class EasingTypeConverter : TypeConverter
     {
-        public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
+        public override bool CanConvertFrom(ITypeDescriptorContext? context, Type sourceType)
         {
             return sourceType == typeof(string);
         }
 
-        public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
+        public override object ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object value)
         {
             return Easing.Parse((string)value);
         }

src/Avalonia.Animation/IAnimation.cs

@@ -12,7 +12,7 @@ namespace Avalonia.Animation
         /// <summary>
         /// Apply the animation to the specified control and run it when <paramref name="match" /> produces <c>true</c>.
         /// </summary>
-        IDisposable Apply(Animatable control, IClock clock, IObservable<bool> match, Action onComplete = null);
+        IDisposable Apply(Animatable control, IClock? clock, IObservable<bool> match, Action? onComplete = null);
 
         /// <summary>
         /// Run the animation on the specified control.

src/Avalonia.Animation/IAnimationSetter.cs

@@ -2,7 +2,7 @@ namespace Avalonia.Animation
 {
     public interface IAnimationSetter
     {
-        AvaloniaProperty Property { get; set; }
-        object Value { get; set; }
+        AvaloniaProperty? Property { get; set; }
+        object? Value { get; set; }
     }
 }

src/Avalonia.Animation/IAnimator.cs

@@ -11,11 +11,11 @@ namespace Avalonia.Animation
         /// <summary>
         /// The target property.
         /// </summary>
-        AvaloniaProperty Property {get; set;}
+        AvaloniaProperty? Property {get; set;}
 
         /// <summary>
         /// Applies the current KeyFrame group to the specified control.
         /// </summary>
-        IDisposable Apply(Animation animation, Animatable control, IClock clock, IObservable<bool> match, Action onComplete);
+        IDisposable? Apply(Animation animation, Animatable control, IClock? clock, IObservable<bool> match, Action? onComplete);
     }
 }

src/Avalonia.Animation/ITransition.cs

@@ -10,7 +10,7 @@ namespace Avalonia.Animation
         /// <summary>
         /// Applies the transition to the specified <see cref="Animatable"/>.
         /// </summary>
-        IDisposable Apply(Animatable control, IClock clock, object oldValue, object newValue);
+        IDisposable Apply(Animatable control, IClock clock, object? oldValue, object? newValue);
 
         /// <summary>
         /// Gets the property to be animated.

src/Avalonia.Animation/IterationCount.cs

@@ -97,7 +97,7 @@ namespace Avalonia.Animation
         /// </summary>
         /// <param name="o">The object with which to test equality.</param>
         /// <returns>True if the objects are equal, otherwise false.</returns>
-        public override bool Equals(object o)
+        public override bool Equals(object? o)
         {
             if (o == null)
             {

src/Avalonia.Animation/IterationCountTypeConverter.cs

@@ -6,12 +6,12 @@ namespace Avalonia.Animation
 {
     public class IterationCountTypeConverter : TypeConverter
     {
-        public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
+        public override bool CanConvertFrom(ITypeDescriptorContext? context, Type sourceType)
         {
             return sourceType == typeof(string);
         }
 
-        public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
+        public override object ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object value)
         {
             return IterationCount.Parse((string)value);
         }

src/Avalonia.Animation/KeyFrame.cs

@@ -19,7 +19,7 @@ namespace Avalonia.Animation
     {
         private TimeSpan _ktimeSpan;
         private Cue _kCue;
-        private KeySpline _kKeySpline;
+        private KeySpline? _kKeySpline;
 
         public KeyFrame()
         {
@@ -79,7 +79,7 @@ namespace Avalonia.Animation
         /// Gets or sets the KeySpline of this <see cref="KeyFrame"/>.
         /// </summary>
         /// <value>The key spline.</value>
-        public KeySpline KeySpline
+        public KeySpline? KeySpline
         {
             get
             {

src/Avalonia.Animation/KeySplineTypeConverter.cs

@@ -12,12 +12,12 @@ namespace Avalonia.Animation
     /// </summary>
     public class KeySplineTypeConverter : TypeConverter
     {
-        public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
+        public override bool CanConvertFrom(ITypeDescriptorContext? context, Type sourceType)
         {
             return sourceType == typeof(string);
         }
 
-        public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
+        public override object ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object value)
         {
             return KeySpline.Parse((string)value, CultureInfo.InvariantCulture);
         }

src/Avalonia.Animation/Transition.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Avalonia.Animation.Easings;
 
 namespace Avalonia.Animation
@@ -8,7 +9,7 @@ namespace Avalonia.Animation
     /// </summary>
     public abstract class Transition<T> : AvaloniaObject, ITransition
     {
-        private AvaloniaProperty _prop;
+        private AvaloniaProperty? _prop;
 
         /// <summary>
         /// Gets or sets the duration of the transition.
@@ -26,7 +27,8 @@ namespace Avalonia.Animation
         public Easing Easing { get; set; } = new LinearEasing();
 
         /// <inheritdocs/>
-        public AvaloniaProperty Property
+        [DisallowNull]
+        public AvaloniaProperty? Property
         {
             get
             {
@@ -42,16 +44,25 @@ namespace Avalonia.Animation
             }
         }
 
+        AvaloniaProperty ITransition.Property
+        {
+            get => Property ?? throw new InvalidOperationException("Transition has no property specified.");
+            set => Property = value;
+        }
+
         /// <summary>
         /// Apply interpolation to the property.
         /// </summary>
         public abstract IObservable<T> DoTransition(IObservable<double> progress, T oldValue, T newValue);
 
         /// <inheritdocs/>
-        public virtual IDisposable Apply(Animatable control, IClock clock, object oldValue, object newValue)
+        public virtual IDisposable Apply(Animatable control, IClock clock, object? oldValue, object? newValue)
         {
-            var transition = DoTransition(new TransitionInstance(clock, Delay, Duration), (T)oldValue, (T)newValue);
+            if (Property is null)
+                throw new InvalidOperationException("Transition has no property specified.");
+
+            var transition = DoTransition(new TransitionInstance(clock, Delay, Duration), (T)oldValue!, (T)newValue!);
             return control.Bind<T>((AvaloniaProperty<T>)Property, transition, Data.BindingPriority.Animation);
         }
     }
-}
+}

src/Avalonia.Animation/TransitionInstance.cs

@@ -10,11 +10,11 @@ namespace Avalonia.Animation
     /// </summary>
     internal class TransitionInstance : SingleSubscriberObservableBase<double>, IObserver<TimeSpan>
     {
-        private IDisposable _timerSubscription;
+        private IDisposable? _timerSubscription;
         private TimeSpan _delay;
         private TimeSpan _duration;
         private readonly IClock _baseClock;
-        private TransitionClock _clock;
+        private TransitionClock? _clock;
 
         public TransitionInstance(IClock clock, TimeSpan delay, TimeSpan duration)
         {
@@ -67,7 +67,7 @@ namespace Avalonia.Animation
         protected override void Unsubscribed()
         {
             _timerSubscription?.Dispose();
-            _clock.PlayState = PlayState.Stop;
+            _clock!.PlayState = PlayState.Stop;
         }
 
         protected override void Subscribed()

src/Avalonia.Visuals/Animation/Animators/BaseBrushAnimator.cs

@@ -38,8 +38,8 @@ namespace Avalonia.Animation.Animators
         }
 
         /// <inheritdoc/>
-        public override IDisposable Apply(Animation animation, Animatable control, IClock clock,
-            IObservable<bool> match, Action onComplete)
+        public override IDisposable? Apply(Animation animation, Animatable control, IClock? clock,
+            IObservable<bool> match, Action? onComplete)
         {
             if (TryCreateCustomRegisteredAnimator(out var animator)
                 || TryCreateGradientAnimator(out animator)
@@ -135,9 +135,8 @@ namespace Avalonia.Animation.Animators
 
         private bool TryCreateCustomRegisteredAnimator([NotNullWhen(true)] out IAnimator? animator)
         {
-            if (_brushAnimators.Count > 0)
+            if (_brushAnimators.Count > 0 && this[0].Value?.GetType() is Type firstKeyType)
             {
-                var firstKeyType = this[0].Value.GetType();
                 foreach (var (match, animatorType) in _brushAnimators)
                 {
                     if (!match(firstKeyType))

src/Avalonia.Visuals/Animation/Animators/GradientBrushAnimator.cs

@@ -58,6 +58,11 @@ namespace Avalonia.Animation.Animators
 
         public override IDisposable BindAnimation(Animatable control, IObservable<IGradientBrush?> instance)
         {
+            if (Property is null)
+            {
+                throw new InvalidOperationException("Animator has no property specified.");
+            }
+
             return control.Bind((AvaloniaProperty<IBrush?>)Property, instance, BindingPriority.Animation);
         }
 

src/Avalonia.Visuals/Animation/Animators/SolidColorBrushAnimator.cs

@@ -24,6 +24,11 @@ namespace Avalonia.Animation.Animators
 
         public override IDisposable BindAnimation(Animatable control, IObservable<ISolidColorBrush?> instance)
         {
+            if (Property is null)
+            {
+                throw new InvalidOperationException("Animator has no property specified.");
+            }
+
             return control.Bind((AvaloniaProperty<IBrush?>)Property, instance, BindingPriority.Animation);
         }
     }

src/Avalonia.Visuals/Animation/Animators/TransformAnimator.cs

@@ -14,10 +14,15 @@ namespace Avalonia.Animation.Animators
         DoubleAnimator? _doubleAnimator;
 
         /// <inheritdoc/>
-        public override IDisposable? Apply(Animation animation, Animatable control, IClock clock, IObservable<bool> obsMatch, Action onComplete)
+        public override IDisposable? Apply(Animation animation, Animatable control, IClock? clock, IObservable<bool> obsMatch, Action? onComplete)
         {
             var ctrl = (Visual)control;
 
+            if (Property is null)
+            {
+                throw new InvalidOperationException("Animator has no property specified.");
+            }
+
             // Check if the Target Property is Transform derived.
             if (typeof(Transform).IsAssignableFrom(Property.OwnerType))
             {