Steps to reproduce:

1. Create CT of name and identifier "author" and add single Text line fieldtype with name and identifier "name".
2. Create CT of name and identifier "book" and add single Text line fieldtype with name and identifier "title", add Content relation (single) fieldtype with name and identifier "author" and allowed CT as "author" created in step 1.
3. Create some "author" Contents.
4. Create "book" Content with "author" relation.
5. Remove "author" fieldtype from "book" CT.
6. Edit created "book" from step 4. and publish.

Result:
Argument 1 passed to eZ\Publish\API\Repository\Values\ContentType\ContentType::hasFieldDefinition() must be of the type string, null given exception.

Expected result:
No exception.

Notes:

1. The same behaviour also occurs for Content relation (multiple) fieldtype.
2. Problem doesn't occur in v2.5.