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. Save the 'Folder' CT.
6. Go to Content / Content structure.
7. Start creating a new Folder CT.

Actual result:

See attached screenshots.

Creating a new Folder content with taxonomy entry assignment fieldtype causes an error 500

An error has occurred resolving the options of the form "Ibexa\Taxonomy\Form\Type\TaxonomyEntryAssignmentFieldType": The option "taxonomy" with value null is expected to be of type "string", but is of type "null". 

Expected result:

No exception occurs.

Note:
If we change 'Product categories' (default option) to 'Tags' in field 'Taxonomy' for Taxonomy Entry Assignment field type then no exception occurs.