Steps to reproduce:

1. Log in to backoffice.
2. Go to Content / Content Types / Content.
3. Edit content type 'Folder'.
4. Add 'Taxonomy Entry Assignment' field type to field definition by drag and drop.
5. Change 'Product categories' to 'Tags' for 'Taxonomy' field in taxonomy entry assignment field type.
6. Save the 'Folder' CT.
7. Go to Content / Content structure.
8. Add new Folder CT.
9. Delete created CT Folder by an option 'Delete' in Content-tree.
10. Repeat steps 2 and 3.
11. Delete taxonomy entry assignment field type from Folder CT.
12. Save the Content.

Actual result:

See attached screenshots.

Cannot delete taxonomy field type from CT when CT based on this field type exist in the system.

Call to a member function getFieldTypeIdentifier() on null

Expected result:

Implemented validation and notification which prevent from deleting the content with field type which is still in use.