fix: Workaround non-UIElement UIViews in Grid layout (backport #18470)

[mergify]

GitHub Issue (If applicable): closes #17997, closes #14013

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

Focusing a text input on iOS causes the framework to temporarily add a non-UIElement child to the parent's Grid, which is then surfaced to the Grid as a null

What is the new behavior?

No longer crashing. This is a workaround, ideally we would want to ensure that Children as a UIElementCollection only reflects the actual UIElements in the ChildrenShadow. This however means more complex logic to properly handle indexes, etc. This workaround seems to be sufficient for now to handle the cases we encountered. Created #18474 to track this

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Validated PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
• Commits must be following the Conventional Commits specification.

Other information

Internal Issue (If applicable):

---

This is an automatic backport of pull request #18470 done by [Mergify](https://mergify.com).