- Make sure you are using the latest version of MIT App Inventor
- Make sure you have the latest companion installed. (Check under help -> companion information on the designer page.
- Use your app and let the error happen.
- Open the menu for the companion (usually three vertical dots at the lower right) and choose "Send error report"
You will see a message with an error report code.
Please then post to the forum, describing your error again and include the error report code, together with your App Inventor login ID and the name of your app.