[CQ] fix nullability problems for flutter/actions

[pq]

Fix nullability problems for all the actions in src/io/flutter/actions/.

See #8291.

If we pursue #8292, we could mark src/io/flutter/actions/ as null-"clean".

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[helin24]

What's with the DataFlowIssue marker? Is it usually this situation in our code? https://stackoverflow.com/questions/76761165/how-to-supress-nullpointer-warning-in-intellij

[pq]

Exactly. It's really unfortunate that the label is so general. It'd be way more useful IMO if it said "nullability issue" or similar.

As a rule, I try and avoid using it but in some cases (when for example, you're interacting w/ unannotated libraries) there's not a great alternative. (I'm on the fence about whether asserts are any better or just add more noise.)

FWIW: if we ever do get to a null-safe world, I'd love to do another round of cleanup that evaluates all the suppressions and cleans them up.

That may be majorly wishful thinking though!