Make sure that _render() is only called with a valid root object. As in,...

Make sure that _render() is only called with a valid root object. As in, render() can only be validly called once and must not fail on the second call. props SergeyBiryukov, see #19642 for trunk.

git-svn-id: https://core.svn.wordpress.org/trunk@19638 1a063a9b-81f0-0310-95a4-ce76da25c4cd