Fix iOS text selection crash by returning nil range

[importRyan]

Reverts a runtime crashing assertion introduced by #16496. Defensively favors returning a nil text selection range rather than crash. See issue #138464 for crash reports and reproduction recordings.

Without this revert, to avoid a crash hit in a minority of user interactions with a text field, we must degrade experiences for all users by disabling useful iOS text field behaviors (see #138464 for workarounds).

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

Fix iOS text selection crash by returning nil range

[chinmaygarde]

cc @justinmc

[importRyan]

@chinmaygarde Thanks for that bump. Is there a chance at getting this in the review queue soon?

This assertion is one of our most common crashes. Simply deselecting the text on some touches (this PR) is preferable to tossing the whole app session out.

If there's desire to fix the upstream range finding, that makes sense, but we'd love to save that for focused follow-on where there's a test battery.

[jmagman]

@hellohuanlin can you take a look? My concern is that this regresses #16496 and b/149077991, in which case we need a different solution.
flutter/flutter#138464 (comment)

[importRyan]

Honest question: is a regression possible?

Sessions that currently crash will simply now continue, allowing VO/non-VO users to retry input strategies. #16496 stopped a crash when using VoiceOver in a provided sample app. That sample does not crash now.

Thanks for analyzing this and getting it in the queue!

[hellohuanlin]

Honest question: is a regression possible?

Should be safe since it's removing NSAssert

[jmagman]

Should be safe since it's removing NSAssert

@hellohuanlin NSAsserts are on in release mode in Flutter flutter/flutter#157837 (cbracken mentioned he wanted to fix that)

[jmagman]

Formatting issue
https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8732309981621212289/+/u/test:_test:_Check_formatting/stdout

[chinmaygarde]

@cbracken @jmagman Are we still making progress on this one?

[gvozditskiy]

Any progress with this one? Customers aren't very happy with crashes

[importRyan]

CI failure is only on mac_android_aot_engine. I don't get a retry option, so I'll push a commit to trigger again.