fix: ensure modeL_ exists before calling delegate methods (#20115)

This is a speculative fix for a crash we are seeing in `menuDidClose`.  We
can't repro the crash but the traces have it happening in this method
and just by reading through the impl the only part that jumps out as
Might Crash is this `model_` call.  Other methods in the menu controller
check `model_` before using it so it probably makes sense to do that here
as well.

atom/browser/ui/cocoa/atom_menu_controller.mm

@@ -372,7 +372,8 @@ - (void)menuWillOpen:(NSMenu*)menu {
 - (void)menuDidClose:(NSMenu*)menu {
   if (isMenuOpen_) {
     isMenuOpen_ = NO;
-    model_->MenuWillClose();
+    if (model_)
+      model_->MenuWillClose();
     // Post async task so that itemSelected runs before the close callback
     // deletes the controller from the map which deallocates it
     if (!closeCallback.is_null()) {