Fix memory leaks in context menu

[ValentinVignal]

Part of #141198

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.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact "@test-exemption-reviewer" in the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[ValentinVignal]

cc @polina-c

[ValentinVignal]

I couldn't enable the leak tracking in some tests as they would fail because of CupertinoPageTransition and CupertinoFullscreenDialogTransition. It needs #146999 to be merged too

[polina-c]

Thanks. Options are:

1. Wait for Fix janks and memory leaks in CupertinoPageTransition and CupertinoFullscreenDialogTransition #146999 to merge
2. Create a test that will test cover your change
3. Do what bot suggests

[ValentinVignal]

Let's see if #146999 can get merged first

If not, I'll try to write a test

[ValentinVignal]

@polina-c I modified the tests in test: Update the tests