replace document.body (might be undefined)

extension/ui/common/content-error.js

@@ -30,7 +30,7 @@ const CLOSE_ICON = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAEAAAABACAYAAA
 const SINGLE_FILE_UI_ELEMENT_CLASS = singlefile.helper.SINGLE_FILE_UI_ELEMENT_CLASS;
 const ERROR_BAR_TAGNAME = "singlefile-error-bar";
 
-const CSS_PROPERTIES = new Set(Array.from(getComputedStyle(document.body)));
+const CSS_PROPERTIES = new Set(Array.from(getComputedStyle(document.documentElement)));
 
 let errorBarElement;
 

extension/ui/content/content-ui.js

@@ -43,7 +43,7 @@ const LOG_PANEL_DEFERRED_IMAGES_MESSAGE = browser.i18n.getMessage("logPanelDefer
 const LOG_PANEL_FRAME_CONTENTS_MESSAGE = browser.i18n.getMessage("logPanelFrameContents");
 const LOG_PANEL_STEP_MESSAGE = browser.i18n.getMessage("logPanelStep");
 const LOG_PANEL_WIDTH = browser.i18n.getMessage("logPanelWidth");
-const CSS_PROPERTIES = new Set(Array.from(getComputedStyle(document.body)));
+const CSS_PROPERTIES = new Set(Array.from(getComputedStyle(document.documentElement)));
 
 let selectedAreaElement, logsWindowElement;
 createLogsWindowElement();