Changing platform resets AIR version

[kevinfoley]

Last tested with nightly build version 5.3.4.275, but this issue has existed in nightlies for months.

1. Go to Project > Properties
2. In the Platform box, select any AIR version lower than the latest version
3. Change the platform (e.g. from AIR Mobile to AIR)
4. The AIR version immediately gets reset to the latest version