Fix a close system alert error

src/main.cpp

@@ -75,7 +75,9 @@ int main(int argc, char* argv[])
         thread.start();
         w.showFullScreen();
         w.requestLogin();
-        return a.exec();
+        int ret =  a.exec();
+        thread.terminate();
+        return ret;
     } else{
         w.showFullScreen();
         w.requestLogin();