Fix null reference crash when window is closed during drag-and-drop (#18760)

Author: AnastassiaP

native/Avalonia.Native/src/OSX/AvnView.mm

@@ -836,6 +836,8 @@ - (NSDragOperation)triggerAvnDragEvent: (AvnDragEventType) type info: (id <NSDra
 
     auto effects = ConvertDragDropEffects(nsop);
     auto parent = _parent.tryGet();
+    if (!parent)
+      return NSDragOperationNone;
     int reffects = (int)parent->TopLevelEvents
             ->DragEvent(type, point, modifiers, effects,
                     CreateClipboard([info draggingPasteboard], nil),