Android NullPointerException in AlphaTabApiBase._internalCursorUpdateBeat

[AndreiGorelov]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

On a simple MusicXML viewer screen with alphaTab, a single tap on the score sometimes causes a fatal crash:

java.lang.NullPointerException
at alphaTab.AlphaTabApiBase._internalCursorUpdateBeat(AlphaTabApiBase.kt:1192)
at alphaTab.AlphaTabApiBase._cursorUpdateBeat$lambda$0(AlphaTabApiBase.kt:1119)
at alphaTab.AlphaTabApiBase$$ExternalSyntheticLambda0.invoke(D8$$SyntheticClass:0)
at alphaTab.platform.android.AndroidUiFacade.beginInvoke$lambda$0(AndroidUiFacade.kt:348)
at alphaTab.platform.android.AndroidUiFacade$$ExternalSyntheticLambda5.run(D8$$SyntheticClass:0)
at android.os.Handler.handleCallback(Handler.java:942)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loopOnce(Looper.java:226)
at android.os.Looper.loop(Looper.java:313)
at android.app.ActivityThread.main(ActivityThread.java:8810)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:604)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1067)

In my build line AlphaTabApiBase.kt:1192 corresponds to alphaTab/packages/alphatab/src/AlphaTabApiBase.ts:2421:

const relativePosition = this._previousTick - this._currentBeat!.start;

From logs and behavior it looks like the code is invoked asynchronously while so _currentBeat becomes null sometimes.

Expected Behavior

alphaTab should not crash when the user taps on the score, even if cursors are being created/destroyed, player state changes, or resize events are happening in parallel.

Steps To Reproduce

1. Create a simple Android app screen that hosts alphaTab
2. Enable user interaction and cursor in settings, similar to:

val settings = alphaTab.Settings().apply {
            player.playerMode = PlayerMode.EnabledAutomatic
            player.enableCursor = true
            player.enableAnimatedBeatCursor = true
            player.enableUserInteraction = true
            player.scrollMode = ScrollMode.OffScreen
            display.scale  = 0.8
        }

3. Load a MusicXML score
4. Start playback then tap on the notation area a few times
5. On some taps, the app crashes with NullPointerException in AlphaTabApiBase._internalCursorUpdateBeat

Link to jsFiddle, CodePen, Project

No response

Version and Environment

1. alphaTab version: 1.8.1
2. Platform: Android (Kotlin)
3. Devices: Android 14/16 tablet and phone

Platform

Android (Native)

Anything else?

Possible Fix
In my fork I implemented an fix by adding the guard from null pointer exeption. After that the crash disappeared in my app.
My fork commit for reference: AndreiGorelov@e5d3131

[AndreiGorelov]

Sorry for the confusion. I merged the fix only in my fork and used the Fixes keyword with this issue number, which unexpectedly closed the issue here. I didn’t mean to close it in the upstream repo.

[Danielku15]

Yeah, I have to admit that the Android control is not battle-tested yet. Generally I should invest a bit more in the "integration tests" for all platforms.

For this specific issue I think we need to get some threading aspects under control. While your fix is valid, it is only needed because we do not have the exact same single-threaded (event loop) behavior from JavaScript in place.

On the UI thread we would need to cleanly operations on the main dispatcher. And for background workers we would need one dispatcher for them too handling a similar event loop pattern.