A promise chain failed to handle a rejection. Did you forget to '.catch', or did you forget to 'return'?
See https://developer.mozilla.org/Mozilla/JavaScript_code_modules/Promise.jsm/Promise

Date: Tue Aug 16 2016 18:09:40 GMT-0500 (Central Standard Time)
Full Message: null
Full Stack: JS frame :: resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js :: PendingErrors.register :: line 195
JS frame :: resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js :: this.PromiseWalker.completePromise :: line 718
JS frame :: resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js :: Handler.prototype.process :: line 973
JS frame :: resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js :: this.PromiseWalker.walkerLoop :: line 816
JS frame :: resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js :: this.PromiseWalker.scheduleWalkerLoop/< :: line 750