Uncaught TypeError: Illegal invocation
    at T.fn.init.<anonymous> (jquery.module.min.js:2:44823)
    at Y (jquery.module.min.js:2:6326)
    at T.fn.init.html (jquery.module.min.js:2:44634)
    at Gmail.api.tools.add_compose_button (gmail.js:3712:16)
    at extension.js:31:15
    at handler (gmail.js:2573:21)
    at Gmail.api.observe.trigger_dom (gmail.js:2451:13)
    at Gmail.api.tools.insertion_observer (gmail.js:2797:37)
    at MutationObserver.<anonymous> (gmail.js:2710:39)

"use strict";

(() => {
  // src/extension.js
  var loaderId = setInterval(() => {
    if (!window._gmailjs) {
      return;
    }
    clearInterval(loaderId);
    startExtension(window._gmailjs);
  }, 100);
  
  function startExtension(gmail) {
    console.log("Extension loading...");
    window.gmail = gmail;
	
    gmail.observe.on("load", () => {
      const userEmail = gmail.get.user_email();
      console.log("Hello, " + userEmail + ". This is your extension talking!");
	  
      gmail.observe.on("view_email", (domEmail) => {
        console.log("Looking at email:", domEmail);
        const emailData = gmail.new.get.email_data(domEmail);
        console.log("Email data:", emailData);
      });
	  
      gmail.observe.on("compose", (compose) => {
        console.log("New compose window is opened!", compose);
	
	var compose_ref = gmail.dom.composes()[0];		
	gmail.tools.add_compose_button(compose_ref, 'content_html', function() {
		// Code here			
	}, 'Custom Style Classes');

      });
    });
  }
})();