Reply To: Rig UI not appearing in the Blender GUI

There are two things here: 1. the Rig Ui only shows in Pose Mode:

but there still is the issue with the Error message. This appears when trying to move/resize the Editors (at least when they are connected/affect the Action Editor). So, when loading the .blend file open the settings (‘wheel’ icon top right) and uncheck Load UI:

Then you’ll have to setup the workspace yourself, (save the file, so you only have to do this once) but no more Errors: