Fix double dispose of bindings caused by animations. Add assertion for double dispose.

src/Avalonia.Animation/DisposeAnimationInstanceSubject.cs

@@ -2,14 +2,7 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Reactive.Linq;
 using Avalonia.Animation.Animators;
-using Avalonia.Animation.Utils;
-using Avalonia.Collections;
-using Avalonia.Data;
-using Avalonia.Reactive;
 
 namespace Avalonia.Animation
 {
@@ -46,6 +39,7 @@ namespace Avalonia.Animation
         public void OnError(Exception error)
         {
             _lastInstance?.Dispose();
+            _lastInstance = null;
         }
 
         void IObserver<bool>.OnNext(bool matchVal)
@@ -53,12 +47,18 @@ namespace Avalonia.Animation
             if (matchVal != _lastMatch)
             {
                 _lastInstance?.Dispose();
+
                 if (matchVal)
                 {
                     _lastInstance = _animator.Run(_animation, _control, _clock, _onComplete);
                 }
+                else
+                {
+                    _lastInstance = null;
+                }
+
                 _lastMatch = matchVal;
             }
         }
     }
-}
+}

src/Avalonia.Base/PriorityLevel.cs

@@ -3,7 +3,8 @@
 
 using System;
 using System.Collections.Generic;
-using System.Reactive.Disposables;
+using System.Diagnostics;
+using System.Threading;
 using Avalonia.Data;
 
 namespace Avalonia
@@ -181,9 +182,9 @@ namespace Avalonia
 
         private sealed class RemoveBindingDisposable : IDisposable
         {
-            private readonly LinkedListNode<PriorityBindingEntry> _binding;
             private readonly LinkedList<PriorityBindingEntry> _bindings;
             private readonly PriorityLevel _priorityLevel;
+            private LinkedListNode<PriorityBindingEntry> _binding;
 
             public RemoveBindingDisposable(
                 LinkedListNode<PriorityBindingEntry> binding,
@@ -197,11 +198,21 @@ namespace Avalonia
 
             public void Dispose()
             {
-                PriorityBindingEntry entry = _binding.Value;
+                LinkedListNode<PriorityBindingEntry> binding = Interlocked.Exchange(ref _binding, null);
+
+                if (binding == null)
+                {
+                    // Some system is trying to remove binding twice.
+                    Debug.Assert(false);
+
+                    return;
+                }
+
+                PriorityBindingEntry entry = binding.Value;
 
                 if (!entry.HasCompleted)
                 {
-                    _bindings.Remove(_binding);
+                    _bindings.Remove(binding);
 
                     entry.Dispose();