Prefab overrides crash unity with "Child already has a owner, it must be removed first."

[jtsmith1287]

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I tried to repro this the best I could before filing this issue.

If you create a UI prefab that has a flexlayout on the root canvas. Have another UI prefab in a different scene, and add the first prefab. Make some changes on the parent, then attempt to override the child prefab with the change.