How to avoid runtime errors with the companion app
After importing an extension, please restart the companion app. If you like to use an extension on a different screen, in Screen1 additionally drag the extension into the working area.
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.