Fix crasher in debug description
Fix crasher in debug description

Summary: Avoids calling a `stringValue` selector unless it is known to exist

Reviewed By: fejese

Differential Revision: D22843671

fbshipit-source-id: 56b34b2d652ca06f2bd680ffe2a2102858346cb9

facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl