Crash in Streams fragment

Description

The app crashes when we enter the streams fragment while playing audio.

The crash has been probably introduced in cfeb5d9f

2020-03-30 08:27:00.808 24403-24403/org.videolan.vlc.debug E/AndroidRuntime: FATAL EXCEPTION: main
    Process: org.videolan.vlc.debug, PID: 24403
    java.lang.IllegalArgumentException: Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull, parameter cb
        at org.videolan.vlc.PlaybackService.addCallback(Unknown Source:2)
        at org.videolan.vlc.viewmodels.StreamsModel.onServiceChanged(StreamsModel.kt:76)
        at org.videolan.vlc.viewmodels.StreamsModel.access$onServiceChanged(StreamsModel.kt:42)
        at org.videolan.vlc.viewmodels.StreamsModel$1.invokeSuspend(StreamsModel.kt:49)
        at org.videolan.vlc.viewmodels.StreamsModel$1.invoke(Unknown Source:10)
        at kotlinx.coroutines.flow.FlowKt__TransformKt$onEach$$inlined$unsafeTransform$1$2.emit(Collect.kt:138)
        at kotlinx.coroutines.flow.FlowKt__ChannelsKt.emitAllImpl$FlowKt__ChannelsKt(Channels.kt:59)
        at kotlinx.coroutines.flow.FlowKt__ChannelsKt.emitAll(Channels.kt:28)
        at kotlinx.coroutines.flow.FlowKt.emitAll(Unknown Source:1)
    	at kotlinx.coroutines.flow.FlowKt__ChannelsKt$asFlow$$inlined$unsafeFlow$1.colle
2020-03-30 08:27:00.808 24403-24403/org.videolan.vlc.debug E/AndroidRuntime:     at Android.MODEL(sdk_gphone_x86)
        at Android.VERSION(R)
        at Android.FINGERPRINT(google/sdk_gphone_x86/generic_x86_arm:R/RPP1.200123.017/6206785:user/release-keys)

Expected behavior

Actual behavior

Steps to reproduce

1. Open the app with the Audio fragment loaded
2. Play some audio
3. Navigate to the Streams fragment

Screenshot / video

Context

App version

Master

Android version

Device model

App mode

Smartphone