Fix an issue where the persistentContainer ivar isn't set

[crazytonyli]

Changes

The _persistentStoreCoordinator instance variable is never assigned a value in its lazy getter. This is going to be an issue if the getter is called multiple times, which isn't the case now. But I wasted a few hours on this issue when making other changes.

Test instructions

Unit and UI test suite should be able to cover this change.

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.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

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[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

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• Then installing the build number pr19210-9223f19 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[mokagio]

This definitely looks like the kind of tiny bug that would take ages to discover. Sorry you had to go through it, and thank you for shipping a fix. 🙇‍♂️