Avoid completing a non-nullable Completer with a nullable value

[srawlins]

Implicitly passing a nullable value to a Completer of a non-nullable type can lead to surprising null-asserting exceptions. See dart-lang/sdk#53253 for more details. In this PR, we add an explicit null-assertion, which makes this call in-line with the general concepts of null safety.

Fixes flutter/flutter#137294

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 the Flutter Style Guide recently, and have followed its advice.
• 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.

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[srawlins]

Ping for a review; I believe the test failures are unrelated.

[srawlins]

Is there any way to force merge past the existing breakage? I don't have the right authority, maybe?

[keyonghan]

Is there any way to force merge past the existing breakage? I don't have the right authority, maybe?

Unfortunately, cocoon is not allowed with a force merge.

Seems some issue with some package versions: https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8761714141529130593/+/u/dashboard/stdout.

[srawlins]

I can also disable the cocooon flutter customer tests until cocoon is back in a good state.

[CaseyHillers]

@srawlins the issue is fixed at head (based on https://flutter-dashboard.appspot.com/#/build?repo=cocoon&branch=main). Can you rebase your branch?

@keyonghan can we enable the "update branch" GitHub UI for Cocoon?

[keyonghan]

That should be helpful. Filed flutter/flutter#140167 to track.

[srawlins]

Thanks everyone! Let's merge! 😁

[keyonghan]

Same as other repos, we use autosubmit label to auto land. Added the label.