[Android] Fix NullReferenceException on WebAuthenticatorIntermediateActivity

[Px7-941]

Description of Change

This pull request fixes a NullReferenceException in WebAuthenticatorIntermediateActivity.
Complements this merged pull request #12347.

Issues Fixed

Fixes #13302, #13798, #12347

Browser tests

The following browsers have been configured as the default app in the Android operating system.

Browser	CustomTab	Fallback	Login	Logout	Comment	Fix
Chrome	true	false	ok	ok	-	-
Samsung	true	true	ok	ok	-	-
Firefox	true	false	ok	ok	-	-
Brave	true	false	ok	ok	-	-
Edge	true	false	ok	ok	-	-
Vivaldi	true	false	ok	ok	-	-
Opera	false	true	ok	ok	-	-
Tor	true	false	nok	nok	-	-
Mi Browser	false	true	ok	ok	main activity remains hidden	LaunchMode.SingleTask
Duckduckgo	false	true	ok	ok	main activity remains hidden	LaunchMode.SingleTask

For some browsers a correction of the user implementation is necessary. LaunchMode = LaunchMode.SingleTask.
https://github.com/dotnet/docs-maui/blob/main/docs/platform-integration/snippets/shared_1/Platforms/Android/WebAuthActivity.cs#L6

[ghost]

Hey there @Px7-941! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[Px7-941]

@microsoft-github-policy-service agree

[rmarinho]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[Redth]

I don't believe this is the correct or necessary fix. The reason this is getting into a state where that value is null is due to the start of the flow being from a system browser which is not supposed to call back into the intermediate activity.

#15187 Should fix this.

If you want to use custom tabs, you are likely missing some values in your manifest required in newer android versions. See the doc