Fix crasher with external repos
Fix crasher with external repos

`externalGroup.children` is an array of `PBXReference`. 
Force-downcasting it as `[PBXGroup]` can fail if there are any 
non-group objects in the array (which happened in my project, causing 
a crash).

@dmishe @DavidGoldman

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Reviewed-on: https://github.com/bazelbuild/tulsi/pull/30
PiperOrigin-RevId: 167744506
MOS_MIGRATED_REVID=167744506

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