UI: Prevent crash when right clicking on an unloaded image

Author: https://github.com/Gingeh
Commit: 6fd03425b2
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3210
Reviewed-by: https://github.com/tcl3 ✅

UI/Qt/Tab.h

@@ -151,7 +151,8 @@ private:
     URL::URL m_link_context_menu_url;
 
     QMenu* m_image_context_menu { nullptr };
-    Gfx::ShareableBitmap m_image_context_menu_bitmap;
+    QAction* m_image_context_menu_copy_image_action { nullptr };
+    Optional<Gfx::ShareableBitmap> m_image_context_menu_bitmap;
     URL::URL m_image_context_menu_url;
 
     QMenu* m_audio_context_menu { nullptr };