Use weak reference to avoid a retain cycle

[crazytonyli]

Fixes #21039.

See the issue details for how to verify the fix.

Regression Notes

1. Potential unintended areas of impact
   None.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   None.

3. What automated tests I added (or what prevented me from doing so)
   None.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

UI Changes testing checklist: N/A

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr21040-7dbb2e5
	Version	22.7
	Bundle ID	org.wordpress.alpha
	Commit	7dbb2e5
	App Center Build	WPiOS - One-Offs #6267

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr21040-7dbb2e5
	Version	22.7
	Bundle ID	com.jetpack.alpha
	Commit	7dbb2e5
	App Center Build	jetpack-installable-builds #5291

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[mokagio]

Too easy