When configure memberOf plug-in in the web UI, you can select  empty values for Subtree Scope and Exclude Subtree fields. Additionally, when you change setting for the MemberOf plug-in and the Subtree Scope was not previously set, you are not able to click Save Config button unless you set the Subtree Scope. 

389-ds-base-libs-2.4.5-3.module+el9dsrv+21615+bdf90ebc.x86_64
389-ds-base-2.4.5-3.module+el9dsrv+21615+bdf90ebc.x86_64
cockpit-389-ds-2.4.5-3.module+el9dsrv+21615+bdf90ebc.noarch

1. Go to Plugins-> MemberOf 
2. Try to change the Group Attribute setting
3.

You can change setting only after you set Subtree Scope. 

You can save setting without entering Subtree Scope field, becaue it is not mandatory. Additionally, there should not be way to select an empty value for the Subtree Scope and Exclude Subtree fields.