HTML Reporter: Check for undefined testItem in testDone callback

For added robustness, allow the DOM node to be missing at this point
just in case. Extracted from #1391.

Author: step2yeung

src/html-reporter/html.js

@@ -902,16 +902,15 @@ export function escapeText( s ) {
 	} );
 
 	QUnit.testDone( function( details ) {
-		var testTitle, time, testItem, assertList, status,
+		var testTitle, time, assertList, status,
 			good, bad, testCounts, skipped, sourceName,
-			tests = id( "qunit-tests" );
+			tests = id( "qunit-tests" ),
+			testItem = id( "qunit-test-output-" + details.testId );
 
-		if ( !tests ) {
+		if ( !tests || !testItem ) {
 			return;
 		}
 
-		testItem = id( "qunit-test-output-" + details.testId );
-
 		removeClass( testItem, "running" );
 
 		if ( details.failed > 0 ) {