Avoid hitting an assertion when calling Toolbar::showMissingItems() with JUCE_MODAL_LOOPS_PERMITTED=0

modules/juce_gui_basics/widgets/juce_Toolbar.cpp

@@ -543,7 +543,7 @@ void Toolbar::showMissingItems()
     {
         PopupMenu m;
         m.addCustomItem (1, new MissingItemsComponent (*this, getThickness()));
-        m.showMenuAsync (PopupMenu::Options().withTargetComponent (missingItemsButton.get()), nullptr);
+        m.showMenuAsync (PopupMenu::Options().withTargetComponent (missingItemsButton.get()), [] (int) {});
     }
 }