Audio classification example: "Unable to destroy activity" error

[pomon23]

In the audio classification example app, there is an error that happens when attempting to shut down the app:

D/AndroidRuntime(22879): Shutting down VM
E/AndroidRuntime(22879): FATAL EXCEPTION: main
E/AndroidRuntime(22879): Process: com.example.audio_classification, PID: 22879
E/AndroidRuntime(22879): java.lang.RuntimeException: Unable to destroy activity {com.example.audio_classification/com.example.audio_classification.MainActivity}: java.lang.IllegalArgumentException: Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull, parameter recorder
E/AndroidRuntime(22879): 	at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:5675)
E/AndroidRuntime(22879): 	at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:5720)
E/AndroidRuntime(22879): 	at android.app.servertransaction.DestroyActivityItem.execute(DestroyActivityItem.java:44)
E/AndroidRuntime(22879): 	at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:176)
E/AndroidRuntime(22879): 	at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97)
E/AndroidRuntime(22879): 	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2325)
E/AndroidRuntime(22879): 	at android.os.Handler.dispatchMessage(Handler.java:106)
E/AndroidRuntime(22879): 	at android.os.Looper.loop(Looper.java:246)
E/AndroidRuntime(22879): 	at android.app.ActivityThread.main(ActivityThread.java:8633)
E/AndroidRuntime(22879): 	at java.lang.reflect.Method.invoke(Native Method)
E/AndroidRuntime(22879): 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
E/AndroidRuntime(22879): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
E/AndroidRuntime(22879): Caused by: java.lang.IllegalArgumentException: Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull, parameter recorder
E/AndroidRuntime(22879): 	at com.tfliteflutter.tflite_flutter_helper.TfliteFlutterHelperPlugin$createRecordListener$1.onMarkerReached(Unknown Source:2)
E/AndroidRuntime(22879): 	at com.tfliteflutter.tflite_flutter_helper.TfliteFlutterHelperPlugin.onDetachedFromEngine(TfliteFlutterHelperPlugin.kt:94)
E/AndroidRuntime(22879): 	at io.flutter.embedding.engine.FlutterEngineConnectionRegistry.remove(FlutterEngineConnectionRegistry.java:262)
E/AndroidRuntime(22879): 	at io.flutter.embedding.engine.FlutterEngineConnectionRegistry.remove(FlutterEngineConnectionRegistry.java:270)
E/AndroidRuntime(22879): 	at io.flutter.embedding.engine.FlutterEngineConnectionRegistry.removeAll(FlutterEngineConnectionRegistry.java:278)
E/AndroidRuntime(22879): 	at io.flutter.embedding.engine.FlutterEngineConnectionRegistry.destroy(FlutterEngineConnectionRegistry.java:122)
E/AndroidRuntime(22879): 	at io.flutter.embedding.engine.FlutterEngine.destroy(FlutterEngine.java:416)
E/AndroidRuntime(22879): 	at io.flutter.embedding.android.FlutterActivityAndFragmentDelegate.onDetach(FlutterActivityAndFragmentDelegate.java:648)
E/AndroidRuntime(22879): 	at io.flutter.embedding.android.FlutterActivity.release(FlutterActivity.java:622)
E/AndroidRuntime(22879): 	at io.flutter.embedding.android.FlutterActivity.onDestroy(FlutterActivity.java:643)
E/AndroidRuntime(22879): 	at android.app.Activity.performDestroy(Activity.java:8468)
E/AndroidRuntime(22879): 	at android.app.Instrumentation.callActivityOnDestroy(Instrumentation.java:1344)
E/AndroidRuntime(22879): 	at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:5660)
E/AndroidRuntime(22879): 	... 11 more

This results in Android constantly notifying the user that that "audio_classification keeps stopping". To reproduce, just launch the app and close it normally with the back button a few times in a row.