When a RelationList field type is created with selection limit = 0 (which means "unlimited") further items cannot be added in Edit mode (unless the lists is emptied completely)

Steps to reproduce:
1. Create a new Content Type
2. Add a RelationList field, set selection limit = 0
3. Create new Content Item of created type
4. Add a single relation.
5. Publish
6. Go to Edit mode

Result:
It's not possible to add more Relations
Expected:
It's possible to add more Relations, as there is no limit.