NOT NULL constraint failed

Не могу понять что я делаю не так.

Выдает ошибку:

NOT NULL constraint failed: tasks_solutions.created_by_id

models.py

class Task(models.Model):
    title = models.CharField(max_length=255, verbose_name='Название задачи')
    slug = models.SlugField(max_length=255, verbose_name='Url', unique=True)
    created_by = models.ForeignKey(AdvUser, editable=False, on_delete=models.CASCADE)  # editable=False,
    description = models.TextField(verbose_name='Описание задачи')
    created_at = models.DateTimeField(auto_now_add=True, verbose_name='Опубликовано')
    views = models.IntegerField(default=0, verbose_name='Кол-во просмотров')
    category = models.ManyToManyField(Category, blank=True, related_name='category')
    tags = models.ManyToManyField(Tag, blank=True, related_name='tasks')
    is_active = models.BooleanField(default=True, db_index=True, verbose_name='Выводить в списке?')

    def __str__(self):
        return self.title

    def get_absolute_url(self):
        return reverse('tasks:task', kwargs={"slug": self.slug})

    class Meta:
        verbose_name = 'Задача'
        verbose_name_plural = 'Задачи'
        ordering = ['-created_at']

class Solutions(models.Model):
    tasks = models.ForeignKey(Task, on_delete=models.CASCADE, verbose_name='Решания к задачи')
    created_by = models.ForeignKey(AdvUser, editable=False, on_delete=models.CASCADE)
    content = models.TextField(verbose_name='Содержание')
    is_active = models.BooleanField(default=True, db_index=True, verbose_name='Выводить на экран?')
    created_at = models.DateTimeField(auto_now_add=True, db_index=True, verbose_name='Опубликован')

    class Meta:
        verbose_name_plural = 'Решания'
        verbose_name = 'Решение'
        ordering = ['-created_at']

forms.py

from tasks.models import Solutions
from django import forms

class UserTaskForm(forms.ModelForm):
    class Meta:
        model = Solutions
        exclude =('is_active',)
        widgets = {'tasks': forms.HiddenInput,}

views.py

def detail(request, slug):
    task = Task.objects.get(slug=slug)
    solution = Solutions.objects.filter(tasks=task, is_active=True)
    initial = {'tasks': task.id}
    task.views = F('views') + 1
    task.save()
    task.refresh_from_db()
    if request.user.is_authenticated:
        initial['created_by'] = request.user.id
        form_class = UserTaskForm
    form = form_class(initial=initial)
    if request.method == 'POST':
        c_form = form_class(request.POST)
        if c_form.is_valid():
            c_form.tasks = task
            #c_form.created_by = request.user
            c_form.save()
            messages.add_message(request, messages.SUCCESS,
                                 'Решение Добавлено')
        else:
            form = c_form
            messages.add_message(request, messages.WARNING,
                                 'Решения не добавлено')

    context = {'solution': solution, 'task': task, 'form': form}


    return render(request, 'tasks/views.html', context)