Merge pull request #3537 from MarchingCube/fix-weak-event-manager-compact

WeakEventHandlerManager: Ensure that weak refs with collected refs are compacted as well.

src/Avalonia.Base/Utilities/WeakEventHandlerManager.cs

@@ -183,11 +183,16 @@ namespace Avalonia.Utilities
                 for (int c = 0; c < _count; c++)
                 {
                     var r = _data[c];
+
+                    TSubscriber target = null;
+
+                    r.Subscriber?.TryGetTarget(out target);
+
                     //Mark current index as first empty
-                    if (r.Subscriber == null && empty == -1)
+                    if (target == null && empty == -1)
                         empty = c;
                     //If current element isn't null and we have an empty one
-                    if (r.Subscriber != null && empty != -1)
+                    if (target != null && empty != -1)
                     {
                         _data[c] = default;
                         _data[empty] = r;