add nil check to fix #12163

[lonegunmanb]

As @tombuildsstuff pointed out the source of nil pointer, and the purpose of returning an nil pointer was to "de-associating" child subscription, not child resource mgr group, an extra nil check should be necessary and sufficient.
It should fix #12163

[stephybun]

Thanks for this @lonegunmanb. I have just one small suggestion, otherwise LGTM!

[github-actions]

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[github-actions]

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