Steps to reproduce:

1. Open Administration → General -> Other
2. Check field "Group for discovered hosts" with host "Discovered hosts"
3. Press button "Reset defaults"
4. Check field "Group for discovered hosts"

Result:
field "Group for discovered hosts" is empty and can't save form because it's required field
Expected:
field "Group for discovered hosts" with host "Discovered hosts"

Same with the "User group for database down message" field, but at least it is optional field.