java.lang.IllegalStateException: Exception thrown on Scheduler.Worker thread. Add onError handling.

[y2k]

java.lang.IllegalStateException: Exception thrown on Scheduler.Worker thread. Add `onError` handling.
	at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:60)
	at rx.internal.schedulers.ExecutorScheduler$ExecutorSchedulerWorker.run(ExecutorScheduler.java:104)
	at android.os.Handler.handleCallback(Handler.java:751)
	at android.os.Handler.dispatchMessage(Handler.java:95)
	at android.os.Looper.loop(Looper.java:154)
	at android.app.ActivityThread.main(ActivityThread.java:6239)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:895)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:785)
Caused by: rx.exceptions.OnErrorNotImplementedException
	at rx.internal.util.InternalObservableUtils$ErrorNotImplementedAction.call(InternalObservableUtils.java:386)
	at rx.internal.util.InternalObservableUtils$ErrorNotImplementedAction.call(InternalObservableUtils.java:383)
	at rx.internal.util.ActionSubscriber.onError(ActionSubscriber.java:44)
	at rx.observers.SafeSubscriber._onError(SafeSubscriber.java:157)
	at rx.observers.SafeSubscriber.onError(SafeSubscriber.java:120)
	at rx.internal.operators.OperatorFilter$FilterSubscriber.onError(OperatorFilter.java:87)
	at rx.observers.SerializedObserver.onError(SerializedObserver.java:158)
	at rx.observers.SerializedSubscriber.onError(SerializedSubscriber.java:79)
	at rx.internal.operators.OnSubscribeConcatMap$ConcatMapSubscriber.innerError(OnSubscribeConcatMap.java:192)
	at rx.internal.operators.OnSubscribeConcatMap$ConcatMapInnerSubscriber.onError(OnSubscribeConcatMap.java:340)
	at rx.internal.operators.OperatorObserveOn$ObserveOnSubscriber.checkTerminated(OperatorObserveOn.java:276)
	at rx.internal.operators.OperatorObserveOn$ObserveOnSubscriber.call(OperatorObserveOn.java:219)
	at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:55)
	... 8 more
Caused by: kotlin.KotlinNullPointerException
	at y2k.joyreactor.services.ImageService$replaceToCache$1.call(ImageService.kt:46)
	at rx.internal.operators.OperatorMap$MapSubscriber.onNext(OperatorMap.java:66)
	at rx.internal.operators.OperatorMap$MapSubscriber.onNext(OperatorMap.java:74)
	at rx.internal.producers.SingleDelayedProducer.emit(SingleDelayedProducer.java:102)
	at rx.internal.producers.SingleDelayedProducer.setValue(SingleDelayedProducer.java:85)
	at rx.Single$1$1.onSuccess(Single.java:84)
	at y2k.joyreactor.common.ObservableExtensionsKt$ioSingle$1$1.call(ObservableExtensions.kt:82)
	at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:55)
	at rx.internal.schedulers.ExecutorScheduler$ExecutorSchedulerWorker.run(ExecutorScheduler.java:104)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607)
	at java.lang.Thread.run(Thread.java:761)
Caused by: rx.exceptions.OnErrorThrowable$OnNextValue: OnError while emitting onNext value: null
	at rx.internal.operators.OperatorMap$MapSubscriber.onNext(OperatorMap.java:70)
	... 10 more