Object reference not set to an instance of an object error when clicking on Add-Ins > Plug-ins

I just installed the Trados TTS plug-in in Trados Studio 2021 SR1 - 16.1.8.4404, but when I click on Add-Ins > Plug-ins to start using it and to enter my API key, I get the "Object reference not set to an instance of an object" error message. Has anybody at the same problem? Thanks in advance for any help anyone can provide.

Eduardo Berinstein