Impossible to create new Assesment Group if all of them were deleted

[clevermind-me]

User can delete all Assesment Groups, but then it is impossible to create new ones, because there won't be a parent group that is mandatory field for creation.

The only possible way to solve this problem was:

1. to switch ERP to Developer Mode,
2. then make field 'parent group' not mandatory,
3. create Assesment Group that 'is group'=yes
4. then go back to Normal mode and continue work

I suppose that one Assesment Group should be made not removable to exclude such possibility.