fixed issue with JSDOM

Former-commit-id: a9595f1ebefa94a9d56e017b6b2934724d7d3294

lib/single-file/modules/css-matched-rules.js

@@ -129,7 +129,7 @@ this.singlefile.lib.modules.matchedRules = this.singlefile.lib.modules.matchedRu
 
 	function invalidSelector(selectorText, workStylesheet) {
 		workStylesheet.textContent = selectorText + "{}";
-		return !workStylesheet.sheet.cssRules.length;
+		return workStylesheet.sheet ? !workStylesheet.sheet.cssRules.length : workStylesheet.sheet;
 	}
 
 	function getMatchedElementsSelector(doc, selectorInfo, styles, matchedElementsCache) {
@@ -282,7 +282,7 @@ this.singlefile.lib.modules.matchedRules = this.singlefile.lib.modules.matchedRu
 	function invalidDeclaration(declarationText, workStylesheet) {
 		let invalidDeclaration;
 		workStylesheet.textContent = "single-file-declaration { " + declarationText + "}";
-		if (!workStylesheet.sheet.cssRules[0].style.length) {
+		if (workStylesheet.sheet && !workStylesheet.sheet.cssRules[0].style.length) {
 			if (!declarationText.match(REGEXP_VENDOR_IDENTIFIER)) {
 				invalidDeclaration = true;
 			}