Missing content definition in a ListPartSettings throws an exception

[MikeAlhayek]

Describe the bug

If the ListPartSettings.ContainedContentTypeDefinitions contains content type that is deleted "no more definition", the UI throws a null exception.

To Reproduce

Steps to reproduce the behavior:

1. Create Content Type X and Y
2. Add ListPart to Y and in the ListPartSettings settings, assign X as the contained content type.
3. Add new item of Y.
4. Removed the definition of X content type.
5. Try to view/edit the item created in step Y.

Expected behavior

No exception should be thrown and the invalid content type should be ignored.