java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Iterator java.util.List.iterator()' on a null object reference

[cico1980]

Device
Brand: samsung
Model: Galaxy J6 +
Orientation: Vertical
Available RAM: 1.47 GB
Free disk space: 3.28 GB
Operating system
Version: 8.1.0
Orientation: Vertical
With administrator access: No
Abnormal shutdown
Date: April 17, 2019, 22:38:00

Caused by java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Iterator java.util.List.iterator()' on a null object reference
at com.wafflecopter.multicontactpicker.MultiContactPickerAdapter.getItemPosition(MultiContactPickerAdapter.java:140)
at com.wafflecopter.multicontactpicker.MultiContactPickerAdapter.setContactSelected(MultiContactPickerAdapter.java:134)
at com.wafflecopter.multicontactpicker.MultiContactPickerActivity$4.onNext(MultiContactPickerActivity.java:220)
at com.wafflecopter.multicontactpicker.MultiContactPickerActivity$4.onNext(MultiContactPickerActivity.java:211)
at io.reactivex.internal.operators.observable.ObservableFilter$FilterObserver.onNext(ObservableFilter.java:52)
at io.reactivex.internal.observers.DisposableLambdaObserver.onNext(DisposableLambdaObserver.java:58)
at io.reactivex.internal.operators.observable.ObservableObserveOn$ObserveOnObserver.drainNormal(ObservableObserveOn.java:200)
at io.reactivex.internal.operators.observable.ObservableObserveOn$ObserveOnObserver.run(ObservableObserveOn.java:252)
at io.reactivex.android.schedulers.HandlerScheduler$ScheduledRunnable.run(HandlerScheduler.java:109)
at android.os.Handler.handleCallback(Handler.java:790)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:7000)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:441)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1408)