[flutter_appauth] fixed crash if main activity has been destroyed/unbound

[SamCosta1]

Context

We've seen a couple of instances in our crashlytics logs of a crash happening due to the activity object being null when startActivityForResult is called.

Crash log

Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.Activity.startActivityForResult(android.content.Intent, int)' on a null object reference
       at io.crossingthestreams.flutterappauth.FlutterAppauthPlugin.performAuthorization(FlutterAppauthPlugin.java:465)
       at io.crossingthestreams.flutterappauth.FlutterAppauthPlugin.access$300(FlutterAppauthPlugin.java)
       at io.crossingthestreams.flutterappauth.FlutterAppauthPlugin$1.onFetchConfigurationCompleted(FlutterAppauthPlugin.java:331)
       at net.openid.appauth.AuthorizationServiceConfiguration$ConfigurationRetrievalAsyncTask.onPostExecute(AuthorizationServiceConfiguration.java:417)
       at net.openid.appauth.AuthorizationServiceConfiguration$ConfigurationRetrievalAsyncTask.onPostExecute(AuthorizationServiceConfiguration.java:358)
       at android.os.AsyncTask.finish(AsyncTask.java:771)
       at android.os.AsyncTask.access$900(AsyncTask.java:199)
       at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:788)
       at android.os.Handler.dispatchMessage(Handler.java:106)
       at android.os.Looper.loop(Looper.java:233)
       at android.app.ActivityThread.main(ActivityThread.java:8063)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:631)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:978)

Solution

A null check with a new error code

Alternative solutions considered

Possibly a more robust solution would be an exponential backoff waiting for the activity to be re-attached with a timeout. Given the maturity of this library though it seems (from what I can see) this issue is very rare, and adding more complexity seems unneeded. Handling it elegently seems sufficient to me.

[MaikuB]

Thanks for the PR. I left a comment about renaming a variable as ERROR was being repeated. Could you take a look?