It looks like the no such class error after importing an extension more frequently (?) appears while being connected via USB...
See also Search results for 'Import extension usb' - MIT App Inventor Community
@ewpatton probably it makes sense to investigate, why this happens?
My general advice usually is
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.
In case restarting the companion app does not work for you after trying several times, then as it looks like the only way for you to test you app will be after building it using the apk file.
Taifun