title: How do I trace an `EmptyError: no elements in sequence` bug in an angular project
body: <p>I'm working on an Angular project that's throwing an error: <code>core.js:1350 ERROR Error: Uncaught (in promise): EmptyError: no elements in sequence</code> but I can't seem to trace it to any of my app source code.  The stack trace all points to rxjs classes.</p>
  
  <p>Can anyone tell me how I can debug this error so that I can get to the real error in the code? </p>
  
  <pre><code>core.js:1350 ERROR Error: Uncaught (in promise): EmptyError: no elements in sequence
  EmptyError: no elements in sequence
      at new EmptyError (EmptyError.js:28)
      at FirstSubscriber._complete (first.js:154)
      at FirstSubscriber.Subscriber.complete (Subscriber.js:122)
      at MergeMapSubscriber._complete (mergeMap.js:150)
      at MergeMapSubscriber.Subscriber.complete (Subscriber.js:122)
      at MapSubscriber.Subscriber._complete (Subscriber.js:140)
      at MapSubscriber.Subscriber.complete (Subscriber.js:122)
      at ArrayObservable._subscribe (ArrayObservable.js:124)
      at ArrayObservable.Observable._trySubscribe (Observable.js:172)
      at ArrayObservable.Observable.subscribe (Observable.js:160)
      at new EmptyError (EmptyError.js:28)
      at FirstSubscriber._complete (first.js:154)
      at FirstSubscriber.Subscriber.complete (Subscriber.js:122)
      at MergeMapSubscriber._complete (mergeMap.js:150)
      at MergeMapSubscriber.Subscriber.complete (Subscriber.js:122)
      at MapSubscriber.Subscriber._complete (Subscriber.js:140)
      at MapSubscriber.Subscriber.complete (Subscriber.js:122)
      at ArrayObservable._subscribe (ArrayObservable.js:124)
      at ArrayObservable.Observable._trySubscribe (Observable.js:172)
      at ArrayObservable.Observable.subscribe (Observable.js:160)
      at resolvePromise (zone.js:824)
      at resolvePromise (zone.js:795)
      at eval (zone.js:873)
      at ZoneDelegate.invokeTask (zone.js:425)
      at Object.onInvokeTask (core.js:4621)
      at ZoneDelegate.invokeTask (zone.js:424)
      at Zone.runTask (zone.js:192)
      at drainMicroTaskQueue (zone.js:602)
      at &lt;anonymous&gt;
  </code></pre>