Steps to reproduce:

1. Log into Zabbix frontent with super admin user
2. Create SSH agent (most likely works with other items as well) item and save it
3. Open that item for update and change item type to 'Dependent item'
4. Leave 'Master item' field empty and save changes

Result:

1) Form doesn't appear and user cannot fix error;
2) There are two error messages, second one is incorrect:

Field "Master item" is mandatory.

No permissions to referred object or it does not exist!