ENH:model files will not be imported when downloading plugins

Change-Id: I87ca42420f2f21e8f6e117ebaa5cb389203d70c2

src/slic3r/GUI/Plater.cpp

@@ -8067,7 +8067,7 @@ void Plater::import_model_id(wxString download_info)
         p->set_project_filename(wxString(filename));
     }
     else {
-        wxMessageBox(msg);
+        if (!msg.empty()) wxMessageBox(msg);
         return;
     }
 }