Android app crash on empty refresh token - no exception is thrown

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Using version 4.2.0 of flutter_appauth on Samsung Galaxy 7 with OS 12.

To Reproduce:

1) Setup a server that is misconfigured and will return a refresh_token of empty string when responding to the token endpoint.
2) Call appAuth.authorize() followed by appAuth.token() on the server, but surround the call with a try-catch block. Run this on Android.

Result:
App shutdown with FATAL EXCEPTION error as follows:

E/AndroidRuntime(17080): java.lang.IllegalArgumentException: refresh token must not be empty if defined
E/AndroidRuntime(17080): at net.openid.appauth.Preconditions.checkArgument(Preconditions.java:116)
E/AndroidRuntime(17080): at net.openid.appauth.Preconditions.checkNotEmpty(Preconditions.java:68)
E/AndroidRuntime(17080): at net.openid.appauth.Preconditions.checkNullOrNotEmpty(Preconditions.java:89)
E/AndroidRuntime(17080): at net.openid.appauth.TokenResponse$Builder.setRefreshToken(TokenResponse.java:305)
E/AndroidRuntime(17080): at net.openid.appauth.TokenResponse$Builder.fromResponseJson(TokenResponse.java:222)
E/AndroidRuntime(17080): at net.openid.appauth.AuthorizationService$TokenRequestTask.onPostExecute(AuthorizationService.java:688)
E/AndroidRuntime(17080): at net.openid.appauth.AuthorizationService$TokenRequestTask.onPostExecute(AuthorizationService.java:579)
E/AndroidRuntime(17080): at android.os.AsyncTask.finish(AsyncTask.java:771)
E/AndroidRuntime(17080): at android.os.AsyncTask.access$900(AsyncTask.java:199)
E/AndroidRuntime(17080): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:788)
E/AndroidRuntime(17080): at android.os.Handler.dispatchMessage(Handler.java:106)
E/AndroidRuntime(17080): at android.os.Looper.loopOnce(Looper.java:226)
E/AndroidRuntime(17080): at android.os.Looper.loop(Looper.java:313)
E/AndroidRuntime(17080): at android.app.ActivityThread.main(ActivityThread.java:8751)
E/AndroidRuntime(17080): at java.lang.reflect.Method.invoke(Native Method)
E/AndroidRuntime(17080): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:571)
E/AndroidRuntime(17080): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1135)
I/Process (17080): Sending signal. PID: 17080 SIG: 9

Expected:
Catch block to be entered.

In other words, server errors should not cause the app to crash.

Start with the Android path used by appAuth.authorize() and appAuth.token(), then trace the reported TokenResponse.Builder.fromResponseJson and AuthorizationService.TokenRequestTask calls. Reproduce the empty refresh_token response and verify that the error reaches the Dart try-catch block instead of terminating the Android app.

Written by the indexing model from the issue text.