Error "Something Went Wrong" When Configuring Task Actions
When configuring an Assign Task, Assign Flexi-Task, or Request For Approval action, after installation or after upgrading Nintex, the action configuration window might display the following error:
Something went wrong. And in the ULS logs you see: Application error when access /_layouts/15/NintexWorkflow/MultiOutcomeDialog.aspx, Error=Object reference not set to an instance of an object.
Retract and remove all of the Nintex Workflow solutions, and re-run the workflow installer. This will push new solutions to the farm, and verify that all .aspx files have been properly pushed to the SharePoint Hive.
This is caused by part of the Nintex Workflow solution not deploying correctly during either an upgrade, and not pushing all of the updated .aspx files to the SharePoint Hive.