[macos] Avoid panic when callback is None.

This can happen when window is destroyed/created during a call to user
callback as this causes WindowDelegate method to be called.

Instead if the user callback is `None` store the event in
`pending_events`.

src/platform/macos/window.rs

@@ -57,7 +57,7 @@ impl WindowDelegate {
             };
 
             if let Some(events_loop) = state.events_loop.upgrade() {
-                events_loop.user_callback.call_with_event(event);
+                events_loop.call_user_callback_with_event_or_store_in_pending(event);
             }
         }