Hmm...that's interesting. One of my found errors was resolved by restarting the companion because the user upgraded the extension but did not restart it.
I find this in Taifun's webpage:
While starting the companion app, all necessary assets and also the imported extensions of your project will be copied to your device. So in case the no such class error shows up, the extension code to run the extension is not available on your device. Without that code the companion app is not able to execute methods from the extension.
and:
After importing an extension, please restart the companion app.