Our team here at TechGuilds managed to catch a bug related to the WFFM (Web Forms For Marketers) Insert Form wizard.
While going through form insertion in Sitecore; we were able to add an “existing” form without selecting a placeholder which caused a server error (Value cannot be null) when you try to edit your controls list.
This bug was found on Sitecore XP 8.1 rev.151207 (Update-1) using Web Forms for Marketers 8.1 rev.151217.
This has been reported to Sitecore and they have registered the issue as a bug for the latest WFFM version as it's still reproducible on WFFM 8.2. To track the future status of this bug report, please use Sitecore's public reference number 165730.
We have asked Sitecore to provide a patch to fix the issue. Once the patch is available, I will provide a link to download it.
In the next steps, I am going to provide instructions on how to reproduce the issue and its workaround.
Until next time!