fixed a segfaulting bug in wxUniv: invalid assumption that every window has a parent

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@13036 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/univ/winuniv.cpp

@@ -366,7 +366,7 @@ bool wxWindow::Enable(bool enable)
         return FALSE;
 
     // disabled window can't keep focus
-    if ( FindFocus() == this )
+    if ( FindFocus() == this && GetParent() != NULL )
     {
         GetParent()->SetFocus();
     }