When parent node of selected node is null, crash might occur in GetCSSBackgroundColorState. So when parent is null, we should return default value. In this situation, Edge and Blink don't throw error, Gecko shouldn't return error too.
MozReview-Commit-ID: 5C6jSAFkCso
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extra : rebase_source : 0185f9fb5fde92eee5611e6bb95e8676c10b135c
Makoto Kato