Fix crash in wxUniv when destroying MDI child frame

Disassociate child frame menu bar from its parent before destroying it
to avoid using a dangling pointer later, when destroying the parent.

See https://github.com/wxWidgets/wxWidgets/pull/2505

Closes #19253.

src/generic/mdig.cpp

@@ -407,7 +407,15 @@ wxGenericMDIChildFrame::~wxGenericMDIChildFrame()
         parent->WXRemoveChild(this);
 
 #if wxUSE_MENUS
-    delete m_pMenuBar;
+    if ( m_pMenuBar )
+    {
+        // calling WXRemoveChild() above broke the link between the menu bar
+        // and the parent, so we need to also remove it explicitly
+        if ( parent )
+            parent->RemoveChild(m_pMenuBar);
+
+        delete m_pMenuBar;
+    }
 #endif // wxUSE_MENUS
 }