Add the case where the user is not logged in webapp

src/browser/webview/mattermost.js

@@ -12,7 +12,10 @@ Notification = EnhancedNotification; // eslint-disable-line no-global-assign, no
 Reflect.deleteProperty(global.Buffer); // http://electron.atom.io/docs/tutorial/security/#buffer-global
 
 function isReactAppInitialized() {
-  const initializedRoot = document.querySelector('div[data-reactroot]') || document.querySelector('#root.channel-view');
+  const initializedRoot =
+    document.querySelector('#root.channel-view') || // React 16 webapp
+    document.querySelector('#root .signup-team__container') || // React 16 login
+    document.querySelector('div[data-reactroot]'); // Older React apps
   if (initializedRoot === null) {
     return false;
   }