Resolves: tdf#122016 crash accessing deleted GtkSalMenu

Change-Id: Ib6527e17110b55251fea9f0701d0e02f6fa5a9fe
Reviewed-on: https://gerrit.libreoffice.org/65107
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
diff --git a/vcl/unx/gtk/gtksalmenu.cxx b/vcl/unx/gtk/gtksalmenu.cxx
index e504e5d..f9053ce 100644
--- a/vcl/unx/gtk/gtksalmenu.cxx
+++ b/vcl/unx/gtk/gtksalmenu.cxx
@@ -506,6 +506,8 @@ bool GtkSalMenu::ShowNativePopupMenu(FloatingWindow* pWin, const tools::Rectangl
    g_object_unref(mpActionGroup);
    ClearActionGroupAndMenuModel();

    mpFrame = nullptr;

    return true;
#else
    (void)pWin;
@@ -581,6 +583,9 @@ GtkSalMenu::~GtkSalMenu()
        g_object_unref(mpMenuModel);

    maItems.clear();

    if (mpFrame)
        mpFrame->SetMenu(nullptr);
}

bool GtkSalMenu::VisibleMenuBar()