Fix unit tests.

src/Avalonia.Animation/Animatable.cs

@@ -35,7 +35,7 @@ namespace Avalonia.Animation
                 (o, v) => o.Transitions = v);
 
         private Transitions _transitions;
-
+        private bool _isTransitionsSet = false;
         private Dictionary<AvaloniaProperty, IDisposable> _previousTransitions;
 
         /// <summary>
@@ -66,7 +66,7 @@ namespace Avalonia.Animation
         /// <param name="e">The event args.</param>
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            if (Transitions is null ||  e.Priority == BindingPriority.Animation) return;
+            if (_transitions is null || _previousTransitions is null || e.Priority == BindingPriority.Animation) return;
 
             var match = Transitions.FirstOrDefault(x => x.Property == e.Property);
 

tests/Avalonia.Animation.UnitTests/TransitionsTests.cs

@@ -23,7 +23,7 @@ namespace Avalonia.Animation.UnitTests
             {
                 var border = new Border
                 {
-                    Transitions =
+                    Transitions = new Transitions
                     {
                         new DoubleTransition
                         {
@@ -51,7 +51,7 @@ namespace Avalonia.Animation.UnitTests
             {
                 var border = new Border
                 {
-                    Transitions =
+                    Transitions = new Transitions
                     {
                         new DoubleTransition
                         {

tests/Avalonia.LeakTests/TransitionTests.cs

@@ -27,7 +27,7 @@ namespace Avalonia.LeakTests
                 {
                     var border = new Border
                     {
-                        Transitions =
+                        Transitions = new Transitions
                         {
                             new DoubleTransition
                             {