The documentation for using `findChildIndexCallback` recommends using `indexOf`, but that causes [this line](https://github.com/flutter/flutter/blob/05259ca938c9ea27aa551048b690d5a06371a6c0/packages/flutter/lib/src/rendering/sliver_multi_box_adaptor.dart#L259) to throw in debug mode, and when using `SliverList`, it breaks the render.

This PR changes the usage to check if the index is not negative before using it, and changes to return `null` instead if the child wasn't able to be found.

There's the related issue #107123, but this doesn't actually fix it.

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This PR has been updated to add the snippets that were used in the `findChildIndexCallback` comment as examples with proper tests, as well as updating the comment to reference the new examples.