tdf#93407 GrabFocus during dispose

Reproduced with the gen vclplug

Change-Id: Ia8a5c73908f5af47ef55b4431b6e1e5a21609e7d
(cherry picked from commit b556f14f)
Reviewed-on: https://gerrit.libreoffice.org/17705Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

svx/source/sidebar/line/LineWidthControl.cxx

@@ -184,9 +184,9 @@ void LineWidthControl::Initialize()
 
 void LineWidthControl::GetFocus()
 {
-    if(!mbVSFocus)
+    if (!mbVSFocus && maMFWidth)
         maMFWidth->GrabFocus();
-    else
+    else if (maVSWidth)
         maVSWidth->GrabFocus();
 }