OK. I hadn’t used MIT App Inventor before today, but I have now! 
I’ve just tried to use an example app both in the emulator and on my Android device, and managed to get it to work.
I think the error you are seeing here is because of the way you have configured your app on apps.twitter.com.
Go back to the page where you have set up the app on apps.twitter.com. Switch to the Settings tab, and enter a placeholder value in the Callback URL field (e.g. http://localhost.com). Then, click the Update Settings button.
Let me know how this works for you.