Dunno if this is just the demo, but here:
http://demo.jet.geex-arts.com/admin/auth/user/add/
you can add a password but no username. Submitting this results in a user with username None.
Then when you go to http://demo.jet.geex-arts.com/admin/auth/user/ you get a 500 response.
I can reproduce this too.
@VincentVW, @Ismael-VC, Can you put here the error trace back please?
@SalahAdDin I can't provide a backtrace because this doesn't happen with my installation of Django 1.10.5

However the demo is actually missing this widget input field:

Which allows to save:

But since the demo is not running with DEBUG = True, there is no backtrace:

@Ismael-VC Yes, but you can get a traceback from browser's debug console.
Please come to the django-jet Discord server so we can organize if you like:
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@Ismael-VC Yes, but you can get a traceback from browser's debug console.