Django: Why does my Django model get "on_deleate" error

I know that ForeignHey of Django 2.2 require "on_delete" parameter and defined it in my model. Nevertheless, I got the error "TypeError: init() missing 1 required positional argument: 'on_delete' " Please tell me the resolution of this error

I use Python3.7 and Django 2.2 My model is

from django.db import models
from treebeard.mp_tree import MP_Node
from django_model_to_dict.mixins import ToDictMixin

class Project(ToDictMixin,models.Model):
    name=models.CharField(max_length=200)

    def __str__(self):
        return self.name

class Task(MP_Node, ToDictMixin):
    name=models.CharField(max_length=100)
    start=models.DateTimeField()
    end=models.DateTimeField()
    progress=models.PositiveIntegerField(default=0)
    custom_class=models.CharField(max_length=20,null=True, blank=True)
    project = models.ForeignKey(
        Project,
        related_name="tasks",
        on_delete=models.CASCADE,
    )
    def __str__(self):
        return self.name

and error is

File "/Users/gensan/python/basic3_7/lib/python3.7/site- packages/django_model_to_dict/models.py", line 127, in <module>
class Order(models.Model, ToDictMixin):
File "/Users/gensan/python/basic3_7/lib/python3.7/site-packages/django_model_to_dict/models.py", line 134, in Order
customer = models.ForeignKey(to=Customer, verbose_name=_('customer'), related_name='orders')
TypeError: __init__() missing 1 required positional argument: 'on_delete'

Thank you