How To Prevent the Profile Option MO: Operating Unit being set to NULL at Site Level

steps
1.Login to the application as SYSADMIN
2.Responsibility : System Administrator
3.Function: Profiles --> System
4.Choose from the menu: Help --> Diagnostics --> Custom Code --> Personalize
5.Create a new Rule
oSeq : 10
oDescription : MO: Operating Unit
oLevel : Function
oEnabled : [checked]
6.Specify the following Condition:
oTrigger Event : WHEN-VALIDATE-RECORD
oTrigger Object : PROFILE_VALUES
oCondition : :PROFILE_VALUES.PROFILE_OPTION_NAME = 'ORG_ID' AND :PROFILE_VALUES.SITE_VISIBLE_VALUE IS NULL
oProcessing Mode : Not in Enter-Query Mode
7.Assign the following Actions to the Rule:
oSeq : 10
oType : Message
oDescription : MO: Operating Unit
oLanguage : All
oEnabled : [checked]
oMessage Type : Error
oMessage Text : You must specify a profile option value at Site level for the profile option 'MO: Operating Unit'.
8.Save the changes
9.Close the System Profile Values form

Result
An error message will appear if an end user now tries to remove the profile option value.