coverity#1209709 Dereference null return value

Change-Id: Ic5cf8a5f52a7219f32ec2ce9259e5aa0153da9bb

sc/source/ui/app/scmod.cxx

@@ -1893,7 +1893,7 @@ void ScModule::EndReference()
                 pRefDlg->SetActive();
             }
         }
-        else
+        else if (pModalDlg)
             pModalDlg->SetActive();
     }
 }