pressing back arrow in wavegen activity makes app crash

[Wishy-S]

Actual Behaviour

pressing back arrow in WaveGeneratorActivity.java makes the app crash.

Expected Behaviour
App should not crash and back button should work properly.

Steps to reproduce it
Go to the Wavegenerator and press the back arrow from the app bar

LogCat for the issue

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: io.pslab, PID: 7634
    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean io.pslab.activity.WaveGeneratorActivity$ProduceSoundTask.cancel(boolean)' on a null object reference
        at io.pslab.activity.WaveGeneratorActivity.onOptionsItemSelected(WaveGeneratorActivity.java:678)
        at android.app.Activity.onMenuItemSelected(Activity.java:3543)
        at androidx.fragment.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:352)
        at androidx.appcompat.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:264)
        at androidx.appcompat.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:109)
        at androidx.appcompat.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:109)
        at androidx.appcompat.widget.ToolbarWidgetWrapper$1.onClick(ToolbarWidgetWrapper.java:188)
        at android.view.View.performClick(View.java:6597)
        at android.view.View.performClickInternal(View.java:6574)
        at android.view.View.access$3100(View.java:778)
        at android.view.View$PerformClick.run(View.java:25885)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

Screenshots of the issue

2021-10-29.01-44-38.mp4

Would you like to work on the issue?
Yup

[Wishy-S]

One more doubt please tell me some info about the branches codacy-badge , master and dependabot... .
What these branches are all about I am a bit confused.
Thank You!

[CloudyPadmal]

https://github.com/fossasia/pslab-android#branch-policy