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

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

@@ -778,6 +778,8 @@
 
     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),