Excel Template using Groovy

I am trying to export data from a multi-choice field in Code beamer to a single cell with the data separated by commas.

I have used the groovy script from the Code beamer Excel Templates page. https://codebeamer.com/cb/wiki/597447

This can be seen in the attached text file "GroovyNotWorking.txt" see  1/.

I have modified it to use the field I am trying to export see 2/.

The exported excel file has the error message in the relevant cell. see 3/.