duplicate key value violates unique constraint "users_user_username_06e46fe6_uniq"
DETAIL:  Key (username)=() already exists.

class FighterSignUpForm(UserCreationForm):
        class Meta(UserCreationForm.Meta):
            model = User
            fields = ('first_name', 'last_name','email','password1', 'password2',)

        @transaction.atomic
        def save(self):
            user = super().save(commit=False)
            user.is_fighter = True
            user.is_active = False
            user.save()
        return user

class FighterSignUpView(CreateView):

    model = User
    form_class = FighterSignUpForm
    template_name = 'registration/user_signup.html'

    def get_context_data(self, **kwargs):
        kwargs['user_type'] = 'example'
        return super().get_context_data(**kwargs)

    def form_valid(self, form):
        user = form.save()

class UserManager(BaseUserManager):

            use_in_migrations = True

            def _create_user(self, email, password, **extra_fields):
                if not email:
                    raise ValueError('The given email must be set')
                email = self.normalize_email(email)
                user = self.model(email=email, **extra_fields)
                user.set_password(password)
                user.save(using=self._db)
                return user

            def create_user(self, email, password=None, **extra_fields):
                extra_fields.setdefault('is_staff', False)
                extra_fields.setdefault('is_superuser', False)
                return self._create_user(email, password, **extra_fields)

            def create_superuser(self, email, password, **extra_fields):
                extra_fields.setdefault('is_staff', True)
                extra_fields.setdefault('is_superuser', True)

                if extra_fields.get('is_staff') is not True:
                    raise ValueError('Superuser must have is_staff=True.')
                if extra_fields.get('is_superuser') is not True:
                    raise ValueError('Superuser must have is_superuser=True.')

                return self._create_user(email, password, **extra_fields)

        class User(AbstractUser):
            email = models.EmailField(_('Email'), unique=True)

            USERNAME_FIELD = 'email'
            REQUIRED_FIELDS = []

            is_type1 = models.BooleanField(default=False, verbose_name="type1")
            is_type2 = models.BooleanField(default=False, verbose_name="type2")
            is_type3 = models.BooleanField(default=False, verbose_name="type")
            is_confirmed = models.BooleanField(default=False, verbose_name="Has Confirmed Email")

            objects = UserManager()