fix(android): prevent crash if callbackInfo is null in BackgroundWorker (fixes #527)

[jonathanduke]

Fixes #527

This PR prevents a NullPointerException when FlutterCallbackInformation.lookupCallbackInformation(...) returns null in BackgroundWorker.kt.

Cause

If the stored callback handle (retrieved from SharedPreferences) is invalid or stale, lookupCallbackInformation(...) returns null. The existing code attempts to access callbackInfo.callbackLibraryPath without checking for null, which crashes the app.

Fix

Add a null check for callbackInfo and fail gracefully:

if (callbackInfo == null) {
    Log.e(TAG, "Failed to resolve Dart callback for handle $callbackHandle.")
    completer?.set(Result.failure())
    return@ensureInitializationCompleteAsync
}

This should prevent the app from terminating with an error such as those mentioned in the linked issue:
Attempt to read from field 'java.lang.String io.flutter.view.FlutterCallbackInformation.callbackLibraryPath'

EDIT: fixed compilation error on return thanks to @Tulleb

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[Tulleb]

I am facing compilation issue with your fix @jonathanduke:

e: file:///[...]/BackgroundWorker.kt:100:17 'return' is prohibited here.
e: file:///[...]/BackgroundWorker.kt:100:24 Return type mismatch: expected 'com.google.common.util.concurrent.ListenableFuture<androidx.work.ListenableWorker.Result>', actual 'androidx.work.ListenableWorker.Result'.

FAILURE: Build failed with an exception.

[jonathanduke]

I am facing compilation issue with your fix @jonathanduke:

e: file:///[...]/BackgroundWorker.kt:100:17 'return' is prohibited here.
e: file:///[...]/BackgroundWorker.kt:100:24 Return type mismatch: expected 'com.google.common.util.concurrent.ListenableFuture<androidx.work.ListenableWorker.Result>', actual 'androidx.work.ListenableWorker.Result'.

FAILURE: Build failed with an exception.

Thank you, @Tulleb. I was able to update and compile locally.