Fix panic during restarting the bridge

internal/frontend/qt/frontend.go

@@ -208,7 +208,11 @@ func (s *FrontendQt) watchEvents() {
 			s.Qml.ShowWindow()
 		case <-restartBridgeCh:
 			s.Qml.SetIsRestarting(true)
-			s.App.Quit()
+			// watchEvents is started in parallel with the Qt app.
+			// If the event comes too early, app doesn't have to be ready yet.
+			if s.App != nil {
+				s.App.Quit()
+			}
 		case address := <-addressChangedCh:
 			s.Qml.NotifyAddressChanged(address)
 		case address := <-addressChangedLogoutCh: