Handle GetBindingObservable<object>().

src/Avalonia.Base/Reactive/AvaloniaPropertyBindingObservable.cs

@@ -48,13 +48,25 @@ namespace Avalonia.Reactive
         {
             if (e.Property == _property)
             {
-                var typedArgs = (AvaloniaPropertyChangedEventArgs<T>)e;
-                var newValue = e.Sender.GetValue(typedArgs.Property);
+                if (e is AvaloniaPropertyChangedEventArgs<T> typedArgs)
+                {
+                    var newValue = e.Sender.GetValue(typedArgs.Property);
 
-                if (!typedArgs.OldValue.HasValue || !EqualityComparer<T>.Default.Equals(newValue, _value))
+                    if (!typedArgs.OldValue.HasValue || !EqualityComparer<T>.Default.Equals(newValue, _value))
+                    {
+                        _value = newValue;
+                        PublishNext(_value);
+                    }
+                }
+                else
                 {
-                    _value = newValue;
-                    PublishNext(_value);
+                    var newValue = e.Sender.GetValue(e.Property);
+
+                    if (!Equals(newValue, _value))
+                    {
+                        _value = (T)newValue;
+                        PublishNext(_value);
+                    }
                 }
             }
         }