"Cannot read property 'focus' of null" when I am calling iframe command call-custom-oauth-login

Hello, I have problem with iframe integration, I am trying to use iframe in our app and connect it with our SSO (custom oAuth) - I did set up SSO in rocketchat without a problem - logging in working without problem, but when I try to call iframe command ‘call-custom-oauth-login’ I am getting JS error back “Uncaught TypeError: Cannot read property ‘focus’ of null”

Server Setup Information

|Version|3.3.0|
|Apps Engine Version|1.15.0-beta.3411|
Ubuntu 20.04 - Docker
Number of Running Instances: 1
MongoDB Version: 4.0

I am calling:
document.querySelector(’.rocketchat-iframe’).contentWindow.postMessage({
externalCommand: ‘call-custom-oauth-login’,
service: ‘easy’
}, ‘*’)
Logged error:
2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:215 Uncaught TypeError: Cannot read property ‘focus’ of null
at o (2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:215)
at Object.u.showPopup (2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:215)
at u.launchLogin (2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:215)
at Object. (2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:418)
at D (2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:13)
at Object.n [as launchLogin] (2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:13)
at p.requestCredential (2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:418)
at Object.a. [as loginWithEasy] (2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:418)
at call-custom-oauth-login (2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:418)
at 2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:418
o @ 2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:215
u.showPopup @ 2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:215
u.launchLogin @ 2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:215
(anonymous) @ 2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:418
D @ 2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:13
n @ 2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:13
requestCredential @ 2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:418
a. @ 2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:418
call-custom-oauth-login @ 2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:418
(anonymous) @ 2bafe4f78b03b63711c998260272d12bc1f316fc.js?meteor_js_resource=true:418