Summary:
JS codes sometimes report exceptions with `null` details about stack traces, resulting in native `NullPointerException` crashes when parsing the stack traces.
Added null checking for the stack traces in `ExceptionsManager.js`, ensuring that we never call `updateExceptionMessage` with a null stack
Reviewed By: foghina
Differential Revision: D3455783
fbshipit-source-id: 027969afd8a5d6015e97fce0a26626730cfc83a4