Serializing nested objects with many to many relationships give me an internal server error 500

[mustafaelghrib]

I have three models like those:

class Ambulance(models.Model):
    ambulance_id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    ambulance_name = models.CharField(max_length=255)
    forms = models.ManyToManyField(Form)

class Form(models.Model):
    form_id = models.UUIDField(primary_key=True, default=uuid4, editable=False)
    form_title = models.CharField(max_length=255)

class Page(models.Model):
    page_id = models.UUIDField(primary_key=True, default=uuid4, editable=False)
    page_title = models.CharField(max_length=100, default='Untitled Page')
    form = models.ForeignKey(
        Form,
        related_name='pages',
        on_delete=models.CASCADE,
        null=True
    )

And the serializers as follow:

class AmbulanceSerializer(serializers.ModelSerializer):
    forms = FormSerializer(read_only=True, many=True)
    class Meta:
        model = Ambulance
        fields = ['ambulance_id', 'ambulance_name', 'forms']


class FormSerializer(ModelSerializer):
    pages = PageSerializer(many=True, read_only=True)
    class Meta:
        model = Form
        fields = ['form_id', 'form_title', 'pages']


class PageSerializer(ModelSerializer):
    class Meta:
        model = Page
        fields = '__all__'

They were working and give me the nested object that I wanted but after adding many-to-many relationships, it gives me an internal server error 500 and it didn't tell me what the problem was, after debugging I found the problem occurs when calling AmbulanceSerializer(ambulance) or FormSerializer(form)

Could someone give me some hint? and what the problem actually!