Crashes when toggling activities when sound is enabled

[netmackan]

Describe the bug
The app crashes when starting or stopping activities after being away from the main view.

To Reproduce
Steps to reproduce the behavior:

1. Make sound is enabled in Settings of ATimeTracker.
2. Go to Android main view/screen (i.e. making sure ATimeTracker is not in the foreground).
3. Start ATimeTracker.
4. Start or stop an existing activity

Expected behavior
The activity state changes.

Actual behavior
The app crashes.

Smartphone (please complete the following information):

• Samsung Galaxy S5
• OS: Android 9 (LineageOS)

Additional context
From adb logcat:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.start()' on a null object reference
 	at com.markuspage.android.atimetracker.Activities.onListItemClick(Activities.java:1234)
 	at android.app.ListActivity$2.onItemClick(ListActivity.java:319)
 	at android.widget.AdapterView.performItemClick(AdapterView.java:318)

[netmackan]

Workaround:

• Disable sound in Settings