SOLID: A cancel on authentication to be graciously handled

[gjwgit]

Description

When on the Authenticate screen in the browser:

I click cancel and the app is blocked with an exception:

[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Null check operator used on a null value
#0      TokenResponse.idToken (package:solid_auth/src/openid/src/model/token_response.dart:25:66)
#1      Credential.generateLogoutUrl (package:solid_auth/src/openid/src/openid.dart:231:31)
#2      authenticate (package:solid_auth/solid_auth_client.dart:220:35)
<asynchronous suspension>
#3      solidAuthenticate (package:solid/src/solid/authenticate.dart:78:22)
<asynchronous suspension>
#4      _SolidLoginState.build.<anonymous closure> (package:solid/src/solid/login.dart:212:28)
<asynchronous suspension>

It seems the code to capture this is not doing so.

[gjwgit]

I had to replace on () with catch () for it to catch the above error.

I now get the popup but on clicking Ok it remains with the busy animation:

Need to terminate the animation and return to the login screen.

[gjwgit]

I have a solution of sorts - Replacing the failed auth popup with a navigate to the login app (the current widget).

Please review https://github.com/anusii/solid/tree/gjw/38_clean_failed_authenticate