Array of field names - when function executed 2nd time or further the first position of the array is not displayed in the dialog

Hi all,
 
When i execute the attached code (still work in progress) it is supposed to open 3 excel files and display the list of the fields in the dialog box for the user to chose appropriate field. Each time it should display 'N/A' in the drop down menu but it doesnt when i select something in dialog box in 1st run. It must be something with the iteration of the ListofColumns but I am not sure how to resolve it.
Thank you for your help in advance.