Summary:
This is the 6th most common error in our logging, and I just run into it after upgrading XCode. It happens when idb doesn't return any devices.
(because `''.split('\n')` results in `['']` not `[]`)
Reviewed By: jknoxville
Differential Revision: D25368759
fbshipit-source-id: 44b0e3a5dc4cf06ecdf2dff4a5692943e60d4b1d