Fix NullPointerException in AppLog.addEntry

[backwardstruck]

Description:

This pull request addresses a NPE crash that could occur in the AppLog.addEntry method when attempting to call onLog on a null AppLogListener object.

Also contains a small ReadMe update for clarity.

Problem:

The addEntry method iterated through a list of AppLogListener objects without checking for null values. If no listeners were registered, the loop would attempt to call methods on null objects, leading to an NPE.

NullPointerException: Attempt to invoke interface method 'void org.wordpress.android.util.AppLog$AppLogListener.onLog(org.wordpress.android.util.AppLog$T, org.wordpress.android.util.AppLog$LogLevel, java.lang.String)' on a null object reference
    at org.wordpress.android.util.AppLog.addEntry(AppLog.java:308)
    at org.wordpress.android.util.AppLog.d(AppLog.java:139)
    at org.wordpress.android.fluxc.utils.AppLogWrapper.d(AppLogWrapper.kt:8)
    at org.wordpress.android.fluxc.tools.CoroutineEngine.launch(CoroutineEngine.kt:60)
    at org.wordpress.android.fluxc.store.EncryptedLogStore.onAction(EncryptedLogStore.kt:79)
...
(7 additional frame(s) were not displayed)

EventBusException: Invoking subscriber failed
    at org.greenrobot.eventbus.EventBus.handleSubscriberException(EventBus.java:537)
    at org.greenrobot.eventbus.EventBus.invokeSubscriber(EventBus.java:519)
    at org.greenrobot.eventbus.EventBus.invokeSubscriber(EventBus.java:511)
    at org.greenrobot.eventbus.AsyncPoster.run(AsyncPoster.java:46)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
...
(2 additional frame(s) were not displayed)

Solution:

Thanks for the help figuring out what to do here @jd-alexander
I added a simple null check within the loop to ensure we only call onLog on valid listeners. If a null listener is encountered, a warning message is logged to indicate the issue.

It should be noted that the root problem still remains as I was unable to identify it. i.e. why was the listener null in the first place. This solution just addresses the symptom (NPE), not the underlying problem of the absence of the listener.

Notes:

An alternative approach would be to filter out null listeners before iterating. I did not implemented in this PR due to a preference for simpler code but I'd be happy to change it if we feel that's more performant.

[notandyvee]

I don't really have a problem with this fix. Reasonable to null check. The problem is when adding an entry it's tagged as @NonNull. Any idea how that's possible?

[backwardstruck]

I don't really have a problem with this fix. Reasonable to null check. The problem is when adding an entry it's tagged as @NonNull. Any idea how that's possible?

Thanks for the review! That's a good question. Something doesn't add up. Maybe that's pointing to the problem being somewhere else.

[notandyvee]

Change LGTM! I'll let you merge so you can cut the release whenever you are ready @backwardstruck .