ensure sheet is defined

lib/single-file/css-fonts-minifier.js

@@ -70,7 +70,11 @@ this.fontsMinifier = this.fontsMinifier || (() => {
 					discarded: 0
 				}
 			};
-			let pseudoElementsContent = Array.from(doc.querySelectorAll("style")).map(style => getPseudoElementsContent(doc, style.sheet.cssRules)).join("");
+			let pseudoElementsContent = Array.from(doc.querySelectorAll("style")).map(style => {
+				if (style.sheet) {
+					return getPseudoElementsContent(doc, style.sheet.cssRules);
+				}
+			}).join("");
 			doc.querySelectorAll("style").forEach(style => {
 				if (style.sheet) {
 					stats.rules.processed += style.sheet.cssRules.length;