I´m a beginner at Android development and just started using  Android Studio. I recently  updated  this week from android 1.3 to 1.4 but it messed up something, when  I try to run one of the sample programs I get this error message:

Installation failed with message null.
It is possible that this issue is resolved by uninstalling an existing version of the apk if it is present, and then re-installing.

WARNING: Uninstalling will remove the application data!