Add back null check

source

@@ -86755,6 +86755,9 @@ interface <dfn>NavigatorOnLine</dfn> {
    module script">fetching a single module script</span> asynchronously completes with
    <var>result</var>:</p></li>
 
+   <li><p>If <var>result</var> is null, asynchronously complete this algorithm with null, and abort
+   these steps.</p></li>
+
    <li><p>If the <var>top-level module fetch</var> flag is set, <span data-x="fetch the descendants
    of and instantiate a module script">fetch the descendants of and instantiate</span>
    <var>result</var> given <var>destination</var> and <var>visited set</var>. Otherwise,