Issue #8563: PageLayout on_touch_down out of range

[AJRepo]

See issue: https://github.com/kivy/kivy/issues/8563

PageLayout on_touch_down doesn't check for length of self.children. Can get IndexError: list index out of range. #8563

Maintainer merge checklist

• [ ] Title is descriptive/clear for inclusion in release notes.
• [ ] Applied a Component: xxx label.
• [ ] Applied the api-deprecation or api-break label.
• [ ] Applied the release-highlight label to be highlighted in release notes.
• [ ] Added to the milestone version it was merged into.
• [ ] Unittests are included in PR.
• [ ] Properly documented, including versionadded, versionchanged as needed.

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