Resolves: tdf#122059 don't crash with no parent for tabpage

Change-Id: Id27609a86558094e6b53c763aebe77c97cc11672
Reviewed-on: https://gerrit.libreoffice.org/65147
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/window/tabpage.cxx

@@ -61,7 +61,7 @@ void TabPage::ImplInitSettings()
         SetParentClipMode();
         SetPaintTransparent( false );
 
-        if ( IsControlBackground() )
+        if (IsControlBackground() || !pParent)
             SetBackground( GetControlBackground() );
         else
             SetBackground( pParent->GetBackground() );