fixed issue when rule.style is undefined

lib/single-file/rules-minifier.js

@@ -72,7 +72,7 @@ this.rulesMinifier = this.rulesMinifier || (() => {
 							if (rulesData.selectors.has(selector) || doc.querySelector(selector)) {
 								stylesheetContent += rule.cssText;
 								rulesData.selectors.add(selector);
-								if (rule.style.fontFamily) {
+								if (rule.style && rule.style.fontFamily) {
 									rule.style.fontFamily.split(",").forEach(fontFamily => rulesData.fonts.used.add(fontFamily.trim()));
 								}
 							}
@@ -81,7 +81,7 @@ this.rulesMinifier = this.rulesMinifier || (() => {
 						}
 					}
 				} else {
-					if (rule.type == CSSRule.FONT_FACE_RULE) {
+					if (rule.type == CSSRule.FONT_FACE_RULE && rule.style && rule.style.fontFamily) {
 						rulesData.fonts.declared.add(rule.style.fontFamily.trim());
 					}
 					stylesheetContent += rule.cssText;