trying to select the doctype element

scratchpad.js

let tempScope = {};
Components.utils.import("resource:///modules/devtools/Target.jsm", tempScope);
let TargetFactory = tempScope.TargetFactory;

let target = TargetFactory.forTab(gBrowser.selectedTab);
gDevTools.showToolbox(target).then(function(toolbox) {
  toolbox.selectTool("inspector").then(function(inspector) {
    inspector.selection.setNode(window.content.document.doctype);
  });
});