Issue Summary

Disassociating the Category field from a screen in the Custom fields page, will result in removing the Summary field from that particular screen

Steps to Reproduce

1. Locate the Category field in the Customs field
2. Click on Associate to screens 
3. Untick the particular screen

Expected Results

In the screen disassociated, the Category field should be removed

Actual Results

The Summary field is removed instead

Workaround

Navigate to the Screens page and remove the field from the screen directly rather than the Customs field page