Cannot read property 'appendChild' and please suggest well documented site for Jitsi Meet

whatever document is provided by Jitsi Meet is not properly. can you suggest sites who has well documented and in detail .
I found that error when I call IFrame API. please suggest how we can resolve it?
“TypeError: Cannot read property ‘appendChild’ of null
at k._createIFrame (
at new k (
at jeetsiMeet (file:///C:/Users/PrasadJ/Desktop/silver_side_template_825/index.html:28:13)
at HTMLButtonElement.onclick (file:///C:/Users/PrasadJ/Desktop/silver_side_template_825/index.html#:47:42)”