Re-add the ability to return null in ListView.builder

[rrousselGit]

Fixes a possible regression during NNBD migration that removed the ability to return null in ListView.builder's itemBuilder

fixes #108705

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, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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

[goderbauer]

(from triage): Looks like some tests are failing on this, could you take a look and fix those up?

[rrousselGit]

(from triage): Looks like some tests are failing on this, could you take a look and fix those up?

I'd happily try to fix the CI, but the error messages are unreadable

Example:

��������������������������������������������������������

Any thought on how I could get a readable explanation of the problem? Otherwise I'll have a hard time fixing the issue.

[goderbauer]

The failure appears pretty clearly in the log [1]:

03:48 +2804 ~52: /tmp/flutter sdk/packages/flutter/test/widgets/scroll_view_test.dart: ListView.separated supports null items in itemBuilder                                                           
══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following TestFailure was thrown running a test:
Expected: exactly 4 matching nodes in the widget tree
  Actual: _TextFinder:<5 widgets with text "separator" (ignoring offstage widgets):
Text("separator", dependencies: [DefaultSelectionStyle, DefaultTextStyle, MediaQuery]),
Text("separator", dependencies: [DefaultSelectionStyle, DefaultTextStyle, MediaQuery]),
Text("separator", dependencies: [DefaultSelectionStyle, DefaultTextStyle, MediaQuery]), ...>
   Which: is too many
When the exception was thrown, this was the stack:
#4      main.<anonymous closure> (file:///tmp/flutter%20sdk/packages/flutter/test/widgets/scroll_view_test.dart:179:5)
<asynchronous suspension>
<asynchronous suspension>
(elided one frame from package:stack_trace)
This was caught by the test expectation on the following line:
  file:///tmp/flutter%20sdk/packages/flutter/test/widgets/scroll_view_test.dart line 179
The test description was:
  ListView.separated supports null items in itemBuilder
════════════════════════════════════════════════════════════════════════════════════════════════════

[1] https://cirrus-ci.com/task/6670346764943360?logs=main#L801

[rrousselGit]

Thanks! Weird that Github action doesn't render them clearly. I'll look into it

[Piinks]

Thanks for the updates. So we have updated PageView, ListView, and GridView here, and it looks like AnimatedList, ReorderableList are confirmed not relevant.
It also looks like CupertinoTabScaffold specifies in the docs that tabBuilder cannot be null.

For the three cases that are changing to NullableIndexedWidgetBuilder can you add a templated doc that clarifies the behavior when using null to indicate the end of the list? The max scroll extent will not be accurate unless you are at the end of the list for example. This has caused confusion for a lot of folks in the past, so reintroducing this gives us a better chance to document it. :)

[rrousselGit]

Sorry, I missed your message!

I'll update the docs after the Flutter Vikings (1st of September).

[goderbauer]

(triage): @rrousselGit Do you have another chance to look at this now that Vikings is over?

[rrousselGit]

Oops, sorry for the delay. Done

[Piinks]

This LGTM with nits, but it looks like CI is unhappy. Can you rebase with master to see if it resolves the failures?

[Piinks]

Thanks for the updates! The docs test looks unhappy, but I've noted where the issues are below.

[Piinks]

This needs an /// {@endtemplate}

[rrousselGit]

There is one, it is in the paragraph above

This paragraph isn't part of the @template

[rrousselGit]

Oh, my bad. Let me fix that

[Piinks]

Oh, my bad. Let me fix that

No worries, appeasing the CI gods can feel a bit tedious at times. Thanks for sticking with it!

[Piinks]

[auto-submit]

auto label is removed for flutter/flutter, pr: 108706, due to - Please get at least one approved review if you are already a member or two member reviews if you are not a member before re-applying this label. Reviewers: If you left a comment approving, please use the "approve" review action instead.

[auto-submit]

auto label is removed for flutter/flutter, pr: 108706, due to Validations Fail.

[Piinks]

Ah we need a second reviewer, I'll seek one.

[justinmc]

LGTM 👍

[galiperkin]

Changes are reviewed. LGTM.