App crashes when there are no build warnings (alerts)

[focustense]

See pastebin copy:

Exception Info: System.InvalidOperationException: Sequence contains no elements
  at System.Linq.ThrowHelper.ThrowNoElementsException()
  at System.Linq.Enumerable.Max[TSource,TResult](IEnumerable`1 source, Func`2 selector)
  at Focus.Apps.EasyNpc.Build.Preview.AlertsViewModel.GetErrorLevel()
  at Focus.Apps.EasyNpc.Build.Preview.AlertsViewModel.UpdateResults()
  at Focus.Apps.EasyNpc.Build.Preview.AlertsViewModel.<BeginWatching>b__36_1(BuildSettings settings)
  at System.Reactive.AnonymousSafeObserver`1.OnNext(T value)
  at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
  at System.Reactive.HalfSerializer.ForwardOnNext[T](ISink`1 sink, T item, Int32& wip, Exception& error)
  at System.Reactive.Linq.ObservableImpl.TakeUntil`2._.OnNext(TSource value)
  at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
  at System.Reactive.ObserveOnObserverLongRunning`1.Drain()
  at System.Reactive.ObserveOnObserverLongRunning`1.<>c.<.cctor>b__17_0(ObserveOnObserverLongRunning`1 self, ICancelable cancelable)
  at System.Reactive.Concurrency.NewThreadScheduler.<>c__DisplayClass7_0`1.<ScheduleLongRunning>b__0()
  at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
  at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state) --- End of stack trace from previous location --- 
  at System.Threading.ThreadHelper.ThreadStart()  

Specifically, if there are no alerts then Max throws this exception.

Also, the nature of the crash (just shutting down with no error message) highlights that we might need to be a little more careful with NewThreadScheduler, as it's not marshalling exceptions to the UI thread.