updating custom attribute using groovy

Summary:

I am trying to update custom attribute based on the value user selects in smartlist.Basically in the form, user is allowed to choose Approval status.

On saving the form, Custom attribute will be updated based on the selected value. Below code is throwing error. Could you please help as what I am doing wrong.

Content (please ensure you mask any confidential information):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):

Set<String> editedProjects=[]

 println("Firststep")

operation.grid.dataCellIterator("ProjectApprovalStatus").each { DataCell cell -> 

  if(cell.edited)