test existence of APIs before calling getInstance

lib/single-file/single-file-browser.js

@@ -44,12 +44,12 @@ this.SingleFileBrowser = this.SingleFileBrowser || (() => {
 		docHelper: docHelper,
 		htmlMinifier: this.htmlMinifier,
 		serializer: this.serializer,
-		fontsMinifier: this.fontsMinifier.getInstance(cssTree, this.fontPropertyParser, docHelper),
-		fontsAltMinifier: this.fontsAltMinifier.getInstance(cssTree),
-		cssRulesMinifier: this.cssRulesMinifier.getInstance(cssTree),
-		matchedRules: this.matchedRules.getInstance(cssTree),
-		mediasMinifier: this.mediasMinifier.getInstance(cssTree, this.mediaQueryParser),
-		imagesAltMinifier: this.imagesAltMinifier.getInstance(this.srcsetParser)
+		fontsMinifier: this.fontsMinifier && this.fontsMinifier.getInstance(cssTree, this.fontPropertyParser, docHelper),
+		fontsAltMinifier: this.fontsAltMinifier && this.fontsAltMinifier.getInstance(cssTree),
+		cssRulesMinifier: this.cssRulesMinifier && this.cssRulesMinifier.getInstance(cssTree),
+		matchedRules: this.matchedRules && this.matchedRules.getInstance(cssTree),
+		mediasMinifier: this.mediasMinifier && this.mediasMinifier.getInstance(cssTree, this.mediaQueryParser),
+		imagesAltMinifier: this.imagesAltMinifier && this.imagesAltMinifier.getInstance(this.srcsetParser)
 	};
 	const domUtil = {
 		getResourceContent,