Limiting choices on forms field DJANGO, leaving them as they were?

I have a following problem, and would appreciate some help:

the question is on the bottom :)

#models.py
class Task(models.Model):
    worker_on_task = models.ForeignKey(User, related_name='task', on_delete=models.CASCADE, blank=True, default=User)
    #forms.py
class ChageTaskStatusForm(forms.ModelForm):
    STATUSES = (
        ('S2', 'In progress'),
        ('S3', 'Problem with'),
        ('S4', 'Finished'),
    )
    status = forms.CharField(max_length=25, widget=forms.Select(choices=STATUSES))
    class Meta:
        model = Task
        fields = ['status']

And most important:

def task_details(request, pk):
    form = ChageTaskStatusForm(request.POST)
    task = get_object_or_404(Task, pk=pk)
    worker = get_object_or_404(User, id=request.user.id)
    if request.method == 'POST':
        if form.is_valid():
            form.save()
            
            

    context = {
        'task': task,
        'form': form,
    }
    return render(request, 'usertask/task_details.html', context)

I have many other fields in Task model, but I want to change only one, (status), but I get following error:

IntegrityError at /4/task_details/
(1048, "Column 'worker_on_task_id' cannot be null")
Request Method: POST
Request URL:    http://127.0.0.1:8000/4/task_details/
Django Version: 3.2.11
Exception Type: IntegrityError
Exception Value:    
(1048, "Column 'worker_on_task_id' cannot be null")

What am I missing? :/