Disallow Blank Selection in Lookup Column

I have a lookup column that lets the user choose a Person. That person then gets emailed via a button in another column. After this email has gone out, I need to make sure that Person doesn’t get changed.

To accomplish (almost) this, I’ve set the filter on the lookup column options such that the only option you can choose is the option that was already there. So you can’t change the person.

Trouble is, the user can select Blank which messes things up. Is there any way I can disallow Blank as an option in a lookup column?

Dear @Nick_HE,

To be honest, neither I found a way to disable the “blank” option. See below screenshot:
image image
I don’t know your set-up, but a way around that comes to mind is the make the button inactive if thisRow of the people column is blank, meaning the user cannot set the email
image

Thanks. Yeah it’s actually that I need a record of who the email was sent to as there’s some further stuff that happens after that. Just thought of another solution though:

  1. Add a column, “Cached Person”, hidden from all user-facing views.
  2. Have the sendEmail button take 2 actions: (1) cachedPerson = Person (2) send the email
  3. Add an automation listening for changes to Person

.

IF(
  Step1Result.Person.isBlank AND Step1Result.cachedPerson.isNotBlank,
  Step1Result.Person = Step1Result.cachedPerson,
  _NoOp()
)

Basically, if you change it to blank, the automation will change it back to what it was.