make sure document.documentElement exists

lib/single-file/processors/lazy/content/content-lazy-loader.js

@@ -39,10 +39,12 @@ this.singlefile.lib.processors.lazy.content.loader = this.singlefile.lib.process
 
 	return {
 		process: async options => {
-			const maxScrollY = Math.max(document.documentElement.scrollHeight - (document.documentElement.clientHeight * 1.5), 0);
-			const maxScrollX = Math.max(document.documentElement.scrollWidth - (document.documentElement.clientWidth * 1.5), 0);
-			if (scrollY <= maxScrollY && scrollX <= maxScrollX) {
-				return process(options);
+			if (document.documentElement) {
+				const maxScrollY = Math.max(document.documentElement.scrollHeight - (document.documentElement.clientHeight * 1.5), 0);
+				const maxScrollX = Math.max(document.documentElement.scrollWidth - (document.documentElement.clientWidth * 1.5), 0);
+				if (scrollY <= maxScrollY && scrollX <= maxScrollX) {
+					return process(options);
+				}
 			}
 		},
 		resetZoomLevel: () => {