tdf#91671 avoid use of already disposed control

Change-Id: I9cf5feeb2fc0ea91c7684aaf8adc2809907fa5d7
(cherry picked from commit a14d7e3b)

extensions/source/propctrlr/propertyeditor.cxx

@@ -166,7 +166,8 @@ namespace pcr
 
     void OPropertyEditor::GetFocus()
     {
-        m_aTabControl->GrabFocus();
+        if ( m_aTabControl )
+            m_aTabControl->GrabFocus();
     }