Steps to reproduce

Create a model with a nullable foreign key field

class MyModel(models.Model):
    foreign_field = models.ForeignKey(OtherModel, null=True)

class MyModelSerializer(serializers.ModelSerializer):
    foreign_field_attribute = serializers.CharField(source='foreign_field.my_attribute', allow_null=True, required=False)

Expected behavior

While retrieveing an entry with 'foreign_field' set to null, object should be serialized with 'foreign_field_attribute' set to None

Actual behavior

An exception is raised while trying to retrieve an instance with 'foreign_field' set to Null:
'NoneType' object has no attribute 'foreign_field'

The above works as expceted in version 3.6.2, but in version 3.7.3 (I do not know where in between the behavior is changed), raises an AttributeError. Seems like a change in fields.py / get_attribute method is causing this.