[notifications v3.0.0] FATAL EXCEPTION

### Device / Emulator and OS
- Device: Samsung S20
- OS: Android 13

### Describe the bug
App crash in debug and release mode, some notifications are handle, but for unknown reasons, some notifications crash the plugin

### To Reproduce
Simply use the plugin

### Actual behavior
`D/AndroidRuntime(19862): Shutting down VM
E/AndroidRuntime(19862): FATAL EXCEPTION: main
E/AndroidRuntime(19862): Process: net.example.app, PID: 19862
E/AndroidRuntime(19862): java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.String java.lang.CharSequence.toString()' on a null object reference
E/AndroidRuntime(19862): at dk.cachet.notifications.NotificationListener.onNotificationPosted(NotificationListener.java:40)
E/AndroidRuntime(19862): at android.service.notification.NotificationListenerService.onNotificationPosted(NotificationListenerService.java:456)
E/AndroidRuntime(19862): at android.service.notification.NotificationListenerService$MyHandler.handleMessage(NotificationListenerService.java:2352)
E/AndroidRuntime(19862): at android.os.Handler.dispatchMessage(Handler.java:106)
E/AndroidRuntime(19862): at android.os.Looper.loopOnce(Looper.java:226)
E/AndroidRuntime(19862): at android.os.Looper.loop(Looper.java:313)
E/AndroidRuntime(19862): at android.app.ActivityThread.main(ActivityThread.java:8757)
E/AndroidRuntime(19862): at java.lang.reflect.Method.invoke(Native Method)
E/AndroidRuntime(19862): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:571)
E/AndroidRuntime(19862): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1067)
I/Process (19862): Sending signal. PID: 19862 SIG: 9`