fix npe in closeCaptureSession

[emakar]

This is a resubmission of #6620

We have such npe:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.hardware.camera2.CameraCaptureSession.close()' on a null object reference
	at io.flutter.plugins.camera.Camera.closeCaptureSession(Camera.java:1244)
	at io.flutter.plugins.camera.Camera$1.onClosed(Camera.java:372)
	at android.hardware.camera2.impl.CameraDeviceImpl$5.run(CameraDeviceImpl.java:234)
	at android.os.Handler.handleCallback(Handler.java:938)
	at android.os.Handler.dispatchMessage(Handler.java:99)
	at android.os.Looper.loopOnce(Looper.java:210)
	at android.os.Looper.loop(Looper.java:299)
	at android.os.HandlerThread.run(HandlerThread.java:67)

This code is racy, so I've added a simple protection to it and similar places.

Fixes flutter/flutter#121079.

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[stuartmorgan-g]

Thanks for the contribution!

This code is racy

Please describe the race in detail, either here or in the issue. How specifically is it happening, and what are the threads involved?

so I've added a simple protection to it and similar places.

I'm not seeing how this is a fix; if this code is being run simultaneously from two different threads then you've just replaced an NPE with API violations by double-calling methods that violate the camera state flow. Am I missing something about the change?

• I added new tests to check the change I am making, or this PR is test-exempt.

Where is this test?

[stuartmorgan-g]

We've just completed the migration of the plugin code to the flutter/packages repository, as described in https://flutter.dev/go/flutter-plugins-repo-migration, and this repository is now being archived. Unfortunately that means that all in-progress PRs here must be moved to flutter/packages.

Please see our instructions for an explanation of how to move your PR, and if you have any issues moving your PR please don't hesitate to reach out in the #hackers-ecosystem channel in Discord.

Our apologies that your PR was caught in this one-time transition. We're aware that it's disruptive in the short term, and appreciate your help in getting us to a better long-term state!