ensure style is not null

lib/single-file/doc-helper.js

@@ -47,7 +47,7 @@ this.docHelper = this.docHelper || (() => {
 		if (options.removeHiddenElements) {
 			doc.querySelectorAll("html > body *:not(style):not(script):not(link):not(frame):not(iframe):not(object)").forEach(element => {
 				const style = win.getComputedStyle(element);
-				if (element instanceof win.HTMLElement && (element.hidden || style.display == "none" || ((style.opacity === 0 || style.visibility == "hidden") && !element.clientWidth && !element.clientHeight)) && !element.querySelector("iframe, frame, object[type=\"text/html\"][data]")) {
+				if (element instanceof win.HTMLElement && style && (element.hidden || style.display == "none" || ((style.opacity === 0 || style.visibility == "hidden") && !element.clientWidth && !element.clientHeight)) && !element.querySelector("iframe, frame, object[type=\"text/html\"][data]")) {
 					element.setAttribute(REMOVED_CONTENT_ATTRIBUTE_NAME, "");
 				}
 			});
@@ -55,7 +55,7 @@ this.docHelper = this.docHelper || (() => {
 		if (options.compressHTML) {
 			doc.querySelectorAll("*").forEach(element => {
 				const style = win.getComputedStyle(element);
-				if (style.whiteSpace.startsWith("pre")) {
+				if (style && style.whiteSpace.startsWith("pre")) {
 					element.setAttribute(PRESERVED_SPACE_ELEMENT_ATTRIBUTE_NAME, "");
 				}
 			});