After installing StackApplet by using:
sudo apt install stackapplet
The app doesn't open, so, it fixed by:
sudo apt-get install python-appindicator
Now it's open and appears in the indicator-panel, but,
when I try to add my account, the pop-up dialog for login authentication doesn't open,
and the screen becomes dark but nothing else happens:
The screen after pressing Add a new account:

The screen should look like this:

In the browser console this errors comes up on launch:
jquery.js:9835 GET http://stackauth.com/1.0/sites?key=_qlxmEAOH06hLA1_FsZIGQ&jsonp=jQuery111304529615512861749_1478608610966&_=1478608610967 404 (Not Found)send @ jquery.js:9835ajax @ jquery.js:9214(anonymous function) @ (index):438fire @ jquery.js:3147fireWith @ jquery.js:3259ready @ jquery.js:3471completed @ jquery.js:3502
And on pressing Add a new account:
jqueryui.js:167 Uncaught TypeError: Cannot read property 'msie' of undefined(…)width @ jqueryui.js:167create @ jqueryui.js:162overlay @ jqueryui.js:161open @ jqueryui.js:151_init @ jqueryui.js:148_createWidget @ jqueryui.js:30b.(anonymous function).(anonymous function) @ jqueryui.js:28(anonymous function) @ jqueryui.js:29each @ jquery.js:383each @ jquery.js:135b.fn.(anonymous function) @ jqueryui.js:29AddNew @ (index):212onclick @ (index):1
How may I add my account if the dialog can't open, please?