Flutter Plugin: crash_dump.cpp: failed to attach to thread 185: Permission denied

I am developing a Flutter Plugin and a simple piano keyboard layout which calls the Flutter plugin when onKeyPressed and onKeyUp the piano keys. The piano app can run in an emulator, and can produce the sound. However, from time to time, the app would crash with the error message:

F/crash_dump32(22593): crash_dump.cpp:474] failed to attach to thread 185: Permission denied

and in the emulator, a pop-up warning shows System UI isn't responding with options: Close the app or Wait.

If I chose Wait, it will resume to work, until the same thread error repeats itself.

In my implementation, I have a class written in Kotlin that will be run in a thread:

class Synth : Runnable {
    private lateinit var mThread: Thread
    private var mRunning = false
    private var mFreq = 440.0
    private var mAmp = 0.0
    private var mNumKeysDown = 0


    fun start() {
        mThread = Thread(this)
        mRunning = true
        mThread.start()
    }

    fun stop() {
        mRunning = false
    }

    fun keyDown(key: Int): Int {
        mFreq = Math.pow(1.0594630f.toDouble(), key.toDouble() - 69.0) * 440.0
        mAmp = 1.0
        mNumKeysDown += 1
        return mNumKeysDown
    }
}

and the following pieces are how the Synth class is called from the Plugin written in Kotlin:

1. When the Plugin is initiated from a Flutter client:

class PianoPlugin: FlutterPlugin, MethodCallHandler {

fun setup(plugin: PianoPlugin, binaryMessenger: BinaryMessenger) {
      plugin.channel = MethodChannel(channelName)
      plugin.channel.setMethodCallHandler(plugin)
      plugin.synth = Synth()
      plugin.synth.start()              // <- Start the thread
    }

  override fun onMethodCall(@NonNull call: MethodCall, @NonNull result: Result) {
    if (call.method == "getPlatformVersion") {
      result.success(android.os.Build.VERSION.RELEASE)
    } else if (call.method == "onKeyDown"){
      try {
        val arguments: ArrayList<Int> = call.arguments as ArrayList<Int>
        val numKeysDown: Int = synth.keyDown(arguments.get(0) as Int)    // <- Function call

        result.success(numKeysDown)
      } catch (ex: Exception) {
        result.error("100", ex.message, ex.getStackTrace())
      }
}

I wonder if anyone has bumped into similar thread issue can share some insights of the possible causes. Very appreciated!