[bugfix] Fix NullPointerException

[ayu-v0]

What's changed?

#2847

This PR fixes the NullPointerException that occurs when foreach is called when Set is null.

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Add or update API

• I have added the necessary e2e tests and all cases have passed.

[ayu-v0]

Oh,The merge error is caused by the absence of space after if. I will correct it @yuluo-yx

[tomsun28]

hi @ayu-v0 thanks for pr. Due the discuss #2847 , suggest we solved this by find why the getNextCollectMetrics first time return null instead of ignore the NPE.

[ayu-v0]

hi @ayu-v0 thanks for pr. Due the discuss #2847 , suggest we solved this by find why the getNextCollectMetrics first time return null instead of ignore the NPE.

Thank you very much for your review. I shall try to ascertain further the cause of the problem.

[ayu-v0]

I found that when setting the monitoring time to 10 seconds, this situation will occur during debugging. This situation is caused by overlapping of the occurrence time of two monitors, which is rare. So it feels like you just need to avoid being empty when the foreach executes🤔(The current thinking is this) @tomsun28