Relaunch application Xamarin Mac Issue

I create a Mac app. I used Xamarin.Mac
I run the app, then click X to close. It remains in launch bar. I cannot open it from the launch bar. I have to right click the icon on launch bar, chose Quit, then I can launch it again from the file explorer.
I tried to download app example on https://developer.xamarin.com/samples/mac/ProgressBarExample/
and the same above issue