tdf#122990: return if no mpMenuBarContainerWidget in ApplyPersona

Related to patches for tdf#122241

Change-Id: I0ce46a3a362855aaa6c11f375d20c2212f1cb20b
Reviewed-on: https://gerrit.libreoffice.org/66973
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/unx/gtk/gtksalmenu.cxx

@@ -859,6 +859,8 @@ void GtkSalMenu::CreateMenuBarWidget()
 void GtkSalMenu::ApplyPersona()
 {
 #if GTK_CHECK_VERSION(3,0,0)
+    if (!mpMenuBarContainerWidget)
+        return;
     assert(mbMenuBar);
     // I'm dubious about the persona theming feature, but as it exists, lets try and support
     // it, apply the image to the mpMenuBarContainerWidget