[web] Use defensive null check in text editing placeElement

[flutter-zl]

Fixes #178619

Uses defensive null check (?.) instead of null assertion (!) when calling focusWithoutScroll() on focusedFormElement in GloballyPositionedTextEditingStrategy.placeElement().

This prevents a crash that can occur due to a race condition where hasAutofillGroup returns true but focusedFormElement is null by the time it's accessed.

[mdebbar]

I'm concerned this may be putting a band-aid on the real issue. When hasAutofillGroup is true, the expectation is that focusedFormElement is always a real value.

@gemini-code-assist any idea why focusedFormElement would be null here some times?

[gemini-code-assist]

That's a very good concern, @mdebbar. The pull request description clarifies this point: "This prevents a crash that can occur due to a race condition where hasAutofillGroup returns true but focusedFormElement is null by the time it's accessed."

This indicates that while hasAutofillGroup might be true, the focusedFormElement itself can become null due to a timing issue between when hasAutofillGroup is checked and when focusedFormElement is actually accessed. Race conditions like this can be challenging to eliminate entirely, especially in asynchronous environments.

The ?. operator acts as a defensive measure, preventing a null pointer exception (crash) if focusedFormElement happens to be null at that exact moment. While it doesn't prevent the underlying race condition from occurring, it ensures the application remains stable. If this race condition is observed frequently or causes other unexpected behavior, a deeper investigation into the lifecycle and synchronization of focusedFormElement and hasAutofillGroup might be warranted to try and prevent the null state from ever occurring.

[flutter-zl]

I have added a comment explaining the re-entrancy issue.

1. placeElement() checks hasAutofillGroup → true
2. placeForm() does DOM operations (insertBefore, etc.)
3. DOM ops trigger synchronous event handlers that process a pending TextInput.updateConfig
4. applyConfiguration() replaces inputConfiguration with a config where autofillGroup = null
5. Back in placeElement(), focusedFormElement is now null → crash

[mdebbar]

LGTM!

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