ManyToManyField add function is not working!

Using Django
1 
from django.db import models
from ckeditor.fields import RichTextField
from ckeditor_uploader.fields import RichTextUploadingField
from django.db.models.signals import post_save
from django.dispatch import receiver

import uuid
# Create your models here.
import random

class Themes(models.Model):
    theme = models.CharField(max_length=100)
    def __str__(self):
        return self.theme
    class Meta:
        verbose_name = "Mavzu"
        verbose_name_plural = "Mavzular"
        ordering=['id']

class Question(models.Model):
    theme = models.ForeignKey(Themes,verbose_name="Mavzusi:", on_delete=models.CASCADE)
    question = RichTextUploadingField("Savol",config_name='default2')
    def __str__(self):
        return self.question
    class Meta:
        verbose_name = "Savol"
        verbose_name_plural = "Savollar"
        order_with_respect_to = 'theme'
class Answer(models.Model):
    question = models.ForeignKey(Question, on_delete=models.CASCADE)
    text = RichTextUploadingField("Javob:",config_name='ans')
    is_correct = models.BooleanField("To'g'ri javob:", default=False)
    def __str__(self):
        return self.text
    class Meta:
        verbose_name = "Javob"
        verbose_name_plural = "Javoblar"
        order_with_respect_to = 'question'
        
class Option(models.Model):
    themes = models.ManyToManyField(Themes,verbose_name="Mavzular")
    qbytheme=models.CharField(max_length=100,verbose_name="Savollarning qaysi mavzudan qanchaligi tartib bo'yicha")
    questions = models.ManyToManyField(Question,verbose_name="Savollar",blank=True)
    def __str__(self):
        return str(self.id)
        # # self.questions.clear()
        # cn = list(map(int,self.qbytheme.split(',')))
        # for i in range(len(cn)):
        #     cur = self.themes.all()[i]
        #     ques=cur.question_set.all()
        #     if list(ques)!=[]:
        #         ran=random.choices(ques,k=cn[i])
        #         self.questions.add(*ran)
        #         print(self.questions.all())
@receiver(post_save,sender=Option,dispatch_uid="option_post_save")
def update_options(sender,instance,**kwargs):
    c=Question.objects.first()
    instance.questions.add(c)# problem here
    instance.questions.update()
    print(instance.questions.all())