Refactor: Fix critical state management bug and improve null safety

[miladsade96]

Changes:

1. Correcting state management bug
2. Handling null value properly

[google-cla]

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[miladsade96]

I have signed the google cla, but still shows me as unsigned. Is there any way to force cla/google to reload or recheck the cla status?

[cpovirk]

I clicked the link for rescanning it from https://github.com/google/guava/pull/8006/checks?check_run_id=50725139081, and now it's showing as passing.

If this PR fixes a bug, please add a test that fails without the PR and passes with it.

[miladsade96]

@cpovirk
I refactored the code but the behavior is the same as before.
Is it necessary to add more tests to the test class?