Form invalidad in Django - ManyToMany relationship - django

Question

I'm trying to insert data in my ManyToMany relationship but i don't get it, my form always is invalidad.
I get this error when i try to save the form "The view apps.alumno.views.SubirActividad didn't return an HttpResponse object. It returned None instead."
Models:
class Alumno(models.Model):
user = models.OneToOneField(User, on_delete=models.CASCADE)
nocontrol = models.CharField(max_length=8)
grupo = models.ForeignKey(Grupo, on_delete=models.CASCADE)
def __str__(self):
return '{}'.format(self.nocontrol)
class Actividad(models.Model):
nombre = models.CharField(max_length=50)
descripcion = models.CharField(max_length=100)
asignatura = models.ForeignKey(Asignatura, on_delete=models.CASCADE)
alumno = models.ManyToManyField(Alumno, through='ActividadAlumno', through_fields=('actividad', 'alumno'),)
def __str__(self):
return '{}'.format(self.nombre)
class ActividadAlumno(models.Model):
actividad = models.ForeignKey(Actividad, on_delete=models.CASCADE)
alumno = models.ForeignKey(Alumno, on_delete=models.CASCADE)
fecha = models.DateField(auto_now_add=False, auto_now=True)
documento = models.FileField(upload_to='asignatura/tarea/')
calificacion = models.IntegerField(null=True)
Form:
class ActividadAlumnoForm(forms.ModelForm):
class Meta:
model = ActividadAlumno
fields = [
'actividad',
'alumno',
'documento',
'calificacion',
]
labels = {
'actividad': 'Fecha de entrega',
'alumno': 'Entregado',
'documento': 'Estado',
'calificacion': 'Calificacion',
}
widgets = {
'actividad': forms.Select(),
'alumno': forms.Select(),
'documento': forms.ClearableFileInput(),
'calificacion': forms.NumberInput(),
}
View:
def SubirActividad(request, id_actividad):
actividad = Actividad.objects.get(id=id_actividad)
if request.method=='POST':
form = ActividadAlumnoForm(request.POST)
if form.is_valid():
form.save()
return redirect('alumno:asignatura_detalle', clave=form.instance.asignatura.clave)
else:
alumno = Alumno.objects.get(user=request.user)
form = ActividadAlumnoForm(initial={'actividad': actividad, 'alumno':alumno})
return render(request, 'alumno/actividad_subir.html', {'form':form})