Not to bring this error up once again in 2023, but I'm experiencing it as well with SU 2021, FredoGhost v2.0a, FredoLib 13.7b, and Sketchucation Toolset v4.3.
After getting the error and restarting SU, the toolbar appears but when trying to open the Studio tool or activating the other limited tools that are available with the trial version, nothing happens, no messages or anything.
Disabled all plugins including Sketchucation v4.3 but except for Fredolib 13.7bb.
Uninstalled FredoGhost.
Started up SU 2021 - installed FredoGhost - got the regular Restart SU message.
Restart SU 2021.
FredoGhost toolbar appears - click on Studio tool and the Studio window opens as it should = baseline.
Open Extension Manager - one by one enabling plugins followed immediately by clicking on the Studio tool.
Enabled Sketchucation v4.3 - click FredoGhost Studio tool - no Studio window opens.
Immed. disable Sketchucation v4.3 - click on FredoGhost Studio tool - no Studio window opens.
Uninstalled FredoGhost.
Started up SU 2021 - installed FredoGhost - got the regular Restart SU message.
Restart SU 2021.
FredoGhost toolbar appears - click on Studio tool and the Studio window opens as it should.
I continued through the list of plugins to see if any others prevented FredoGhost Studio from opening, all good except for Sketchucation is still disabled. I'll leave it disabled for now as I trial Ghost and hopefully if all works well, I will get that Ghost license and a new version Sketchucation will be released that doesn't cause this conflict.