[ui][registration][forms] fix supervisor crash on sign-up

Fixes #568

beat/web/ui/registration/forms.py

@@ -433,7 +433,7 @@ class RegistrationSupervisorForm(forms.Form):
 
         """
         username = self.cleaned_data["username"]
-        if User.objects.get(username__iexact=username).exists():
+        if User.objects.filter(username__iexact=username).exists():
             raise forms.ValidationError(
                 _(u"This username is already taken. Please choose another.")
             )