Cannot add "<Model: Model object (None)>": the value for field "field" is None

Question

I am trying to save an object to my database while adding it to the Many to Many field of another object. I already tried many other solutions from here but nothing worked so far.

Model:

class SellerPost(models.Model):
    post_uuid = models.UUIDField(default=uuid.uuid4, editable=False)
    seller = models.ForeignKey("User", on_delete=models.CASCADE)
    text_content = models.TextField()
    comments = models.ManyToManyField("SellerPostComment", blank=True)

class SellerPostComment(models.Model):
    comment_id = models.IntegerField(primary_key=True)
    post = models.ForeignKey(SellerPost, on_delete=models.CASCADE)
    addressed = models.ForeignKey("User", on_delete=models.CASCADE, null=False, related_name="seller_addressed_comment")
    commenter = models.ForeignKey("User", on_delete=models.CASCADE, null=False)
    content = models.TextField()

View (i cut everything but the essential part that has sth to do with the error):

  post = request.POST["post"]
  post_obj = SellerPost.objects.get(post_uuid=post)
  comment = comment_form.save(commit=False)
  comment.addressed = user
  comment.commenter = request.user
  comment.post = post_obj
  comment.save()

  post_obj.comments.add(comment)
            
  return redirect(index)

class PostCommentForm(forms.ModelForm):
    class Meta:
        model = SellerPostComment
        fields = ("content",)
    
    def save(self, commit=True):
        comment = super(PostCommentForm, self).save(commit=False)
        if commit:
            comment.save()
        return comment

Error:

Cannot add "<SellerPostComment: SellerPostComment object (None)>": the value for field "sellerpostcomment" is None

The form is valid but it just won't save the comment to the M2M field of the post. Thanks in advance!