fix: QTree children type safety #15991

[Tofandel]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Documentation
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch (or v[X] branch)
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on an Electron app
• Any necessary documentation has been added or updated in the docs or explained in the PR's description.

Fixes #15991

[github-actions]

UI Tests Results

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0 suites  0 💤
2 files    0 ❌
2 errors

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Results for commit 37d5b42.

[rstoenescu]

Thanks for contributing!