Nova display value from other table in index

Hi,

I have 2 resources/models and I want to display a value from a resource on a index, this works fine, but then when i go to update the detail from the index, I get an error trying to get property of non object. I don't want this value to be updated though, only displayed on the index. I don't want to make the field but display it on the index if that makes sense.

Any ideas on what I am doing wrong?

    DateTime::make('Apt Time', function () { return $this->appointments->first()->apt_time; })->showOnIndex()->format('DD MMM YYYY HH:mm'),

Thanks