Cover the case where global registry become null registry accidentally

Author: Jayson Chen

dom.bs

@@ -6093,14 +6093,24 @@ algorithm is passed <var ignore>node</var> and <var ignore>oldDocument</var>, as
          <li><p>If <var>inclusiveDescendant</var>'s <a for=tree>parent</a> is null or
          <var>inclusiveDescendant</var>'s <a for=tree>parent</a> is a {{DocumentFragment}} but not
          a {{ShadowRoot}}, then set <var>registry</var> to <var>document</var>'s
-         <a for=Document>custom element registry</a>.
-
-         <li><p>Otherwise, set <var>registry</var> to the result of
-         <a for=/>looking up a custom element registry</a> given <var>inclusiveDescendant</var>'s
-         <a for=tree>parent</a>.
+         <a for=Document>custom element registry</a>'s
+         <a>effective global custom element registry</a>.
+
+         <li>
+          <p>Otherwise, set <var>registry</var> to the
+          <a>effective global custom element registry</a> of the result of
+          <a for=/>looking up a custom element registry</a> given <var>inclusiveDescendant</var>'s
+          <a for=tree>parent</a>.
+
+          <ol>
+           <li><p>If <var>inclusiveDescendant</var>'s <a for=Element>custom element registry</a> is
+           non-null and <var>registry</var> is null, set <var>registry</var> to
+           <var>document</var>'s <a for=Document>custom element registry</a>'s
+           <a>effective global custom element registry</a>.
+          </ol>
 
          <li><p>Set <var>inclusiveDescendant</var>'s <a for=Element>custom element registry</a>
-         to <var>registry</var>'s <a>effective global custom element registry</a>.
+         to <var>registry</var>.
         </ol>
       </ol>
     </ol>