added commit=bool(not app_settings.SIGNUP_FORM_CLASS)

The problem was with custom signup form with some required fields. It simply trys to save the user without adding mandatory fields and which throws "required key error". So this was the simple workaround i figured out without adding a extra setting.

i.e. if someone has a custom signup form then let him save the form manually in   
def signup(self, request, user):
     user.save()

method.