Reset next wxMenu event handler in wxUniv when detaching it from wxMenuBar.

A detached menu must not send its events to the menu bar that it isn't part of
any longer.

Closes #2620.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70410 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/univ/menu.cpp

@@ -1207,6 +1207,10 @@ void wxMenu::Attach(wxMenuBarBase *menubar)
 
 void wxMenu::Detach()
 {
+    // After the menu is detached from the menu bar, it shouldn't send its
+    // events to it.
+    SetNextHandler(NULL);
+
     wxMenuBase::Detach();
 }