A better approach to this is a simple Client Script onChange of Assignment Group, to make the variable mandatory: 

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
   if (isLoading || newValue === '') {
      return;
   }

	if (newValue == '302f396287ff22506682653e8bbb3542') {
		g_form.setMandatory('core_sub_category', true);
	}
}

with a similar one onLoad for existing records with this assignment group.  You can add a field message if you want to call attention to the variable value needing populated, but this uses the out of box functionality for mandatory fields, so submitting a record/update with the variable empty will not be possible, and the script will not cause an error if the field/variable is not found.