It may occur that when you are making edits to a NetCommunity form like Donation or Membership forms the drop down showing constituent codes from Raiser's Edge is blank.
This can occur if entries in the Constituent Code table in The Raiser's Edge have invalid characters. The code table entries should be inspected for any invalid special characters. These entries should be deleted or renamed.
Steps to Duplicate
Login to NetCommunity
Access Sire Explorer > Parts
Search for a form part type like Donation Form or Membership Form