Ensure input event is valid in `PopupMenu::activate_item_by_event`

(cherry picked from commit f721b34b4edbab0f88cd2aa671f1231b11aa26af)

scene/gui/popup_menu.cpp

@@ -1812,6 +1812,7 @@ void PopupMenu::scroll_to_item(int p_idx) {
 }
 
 bool PopupMenu::activate_item_by_event(const Ref<InputEvent> &p_event, bool p_for_global_only) {
+	ERR_FAIL_COND_V(p_event.is_null(), false);
 	Key code = Key::NONE;
 	Ref<InputEventKey> k = p_event;