On the page that documents the GameInvitePromptClosed event, this code
local function invitePromptClosed(senderPlayer, recipientIds) -- Handle custom logic for players invited by sender end
implies that GameInvitePromptClosed is supposed to return something. However, this has been changed recently.
The page should be updated to show this change.
Edit: Also wanted to add that in the SocialService Game Invite Demo, it has the same problem.