Fix random NRE inside `Compositor.CommitCore()` callback. (#19173)

* Prevent ContinueWith from scheduling some of rendering callbacks into UI thread. (#19170 fix)

* Add _pendingBatch null check

src/Avalonia.Base/Media/MediaContext.Compositor.cs

@@ -31,7 +31,7 @@ partial class MediaContext
         _pendingCompositionBatches[compositor] = commit;
         commit.Processed.ContinueWith(_ =>
             _dispatcher.Post(() => CompositionBatchFinished(compositor, commit), DispatcherPriority.Send),
-            TaskContinuationOptions.ExecuteSynchronously);
+            CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
         return commit;
     }
     

src/Avalonia.Base/Rendering/Composition/CompositingRenderer.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.ComponentModel;
 using System.Linq;
 using System.Numerics;
+using System.Threading;
 using System.Threading.Tasks;
 using Avalonia.Collections;
 using Avalonia.Collections.Pooled;
@@ -187,7 +188,7 @@ internal class CompositingRenderer : IRendererWithCompositor, IHitTester
             {
                 _queuedSceneInvalidation = false;
                 SceneInvalidated?.Invoke(this, new SceneInvalidatedEventArgs(_root, new Rect(_root.ClientSize)));
-            }, DispatcherPriority.Input), TaskContinuationOptions.ExecuteSynchronously);
+            }, DispatcherPriority.Input), CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
         }
     }
 

src/Avalonia.Base/Rendering/Composition/Compositor.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Threading;
 using System.Threading.Tasks;
 using Avalonia.Animation;
 using Avalonia.Animation.Easings;
@@ -114,7 +115,7 @@ namespace Avalonia.Rendering.Composition
                 if (pending != null)
                     pending.Processed.ContinueWith(
                         _ => Dispatcher.Post(_triggerCommitRequested, DispatcherPriority.Send),
-                        TaskContinuationOptions.ExecuteSynchronously);
+                        CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
                 else
                     _triggerCommitRequested();
             }
@@ -202,10 +203,10 @@ namespace Avalonia.Rendering.Composition
                 {
                     lock (_pendingBatchLock)
                     {
-                        if (_pendingBatch.Processed == t)
+                        if (_pendingBatch?.Processed == t)
                             _pendingBatch = null;
                     }
-                }, TaskContinuationOptions.ExecuteSynchronously);
+                }, CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
                 _nextCommit = null;
                 
                 return commit;