Problem Uploading to Arduino Uno

The beta build of the Arduino IDE you're using is intended primarily for beta testing. You should expect to encounter the occasional bug like this with the beta build. For the most stable experience you will want to use the release versions of the Arduino IDE.

This issue has just been resolved by the Arduino developers:

so if you do want to continue to use the beta build then just download and use the latest version and the problem should be fixed.