[camera] Fix disposed exceptions for the camera example on iOS

[AlexV525]

No CHANGELOG change: only affects the example.
No version change: only affects the example.

Related to flutter/flutter#97199.

Controllers were given to the field aggressively, which cause race conditions when initializing controllers. Now the code will remove the references ASAP when a controller has been disposed.

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 relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• 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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[AlexV525]

Test result without the fix:

[hellohuanlin]

Hi, can you add more details in the description? is it to hide the permission bug from the example project? (if that's the case it is probably better not to hide it so we know there's a bug there)

[AlexV525]

Hi, can you add more details in the description? is it to hide the permission bug from the example project? (if that's the case it is probably better not to hide it so we know there's a bug there)

@hellohuanlin Updated. It's more like a race conditions issue, and it could happen other than the permission issue.

[hellohuanlin]

@hellohuanlin Updated. It's more like a race conditions issue, and it could happen other than the permission issue.

Will this PR hide the permission issue mentioned in flutter/flutter#97199?

[AlexV525]

Will this PR hide the permission issue mentioned in flutter/flutter#97199?

Indeed, we can hold this though.

[stuartmorgan]

No CHANGELOG change: only affects the example.

We generally changelog for example-only changes; see the discussion in the link for that PR item. (Otherwise the bot would be configured not to require the override for that case.)

[Hixie]

What's the status of this PR?

[AlexV525]

What's the status of this PR?

I'm holding this and see if it's still necessary after @hellohuanlin apply fixes to flutter/flutter#97199.

[hellohuanlin]

since this PR hides the bug in the example code, it may not be a good idea to land it