Defensive coding that will possibly fix a crash when scrolling through footprints.

Fixes: lp:1828289
* https://bugs.launchpad.net/kicad/+bug/1828289

cvpcb/cvpcb_mainframe.cpp

@@ -816,6 +816,7 @@ void CVPCB_MAINFRAME::CreateScreenCmp()
         fpframe->Raise();   // Make sure that is visible.
         Raise();            // .. but still we want the focus.
 
+        if( focus )
             focus->SetFocus();
     }