OpenApi AutoSchema inspection produces invalid type `null` on ArrayField(base_field=ChoicesField())

[meteozond]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

Create array field with base_field provided choices:

class A(Model):
	field = ArrayField(base_field=CharField(max_length=256, choices=(1,2,3)))

Expected behavior

properties: {field: {
	type: array
	items: {
		enum: [1, 2, 3]
		type: "string"
	}
}}

or

properties: {field: {
	type: array
	items: {
		enum: [1, 2, 3]
	}
}}

Actual behavior

properties: {
field: {
	type: array
	items: {
		type: null
	}
}}

Note

Looks like choice field processing produces {enum}
https://github.com/encode/django-rest-framework/blob/master/rest_framework/schemas/openapi.py#L256

But list field expects type (self._map_field(field.child).get('type'))
https://github.com/encode/django-rest-framework/blob/master/rest_framework/schemas/openapi.py#L269

[dhaval-mehta]

I have same issue. I would like to fix this issue.