Please do Setting Up SSL on Tomcat to avoid encountering this error. 

javax.net.ssl.SSLHandshakeException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

This error above indicates that the Joget instance does not have any SSL cert installed or has an invalid SSL cert (e.g.: expired cert, improper SSL cert installation, etc.). Typically most public services will enforce mandatory HTTPS, this includes Github APIs.

The Joget server requires a valid SSL cert installed into the web app server's java keystore.

Here are some general guides you can refer to:

• https://dev.joget.org/community/display/DX8/Setting+Up+SSL+on+Tomcat
• https://tomcat.apache.org/tomcat-9.0-doc/ssl-howto.html
• https://www.thesslstore.com/resources/tomcat-ssl-installation/

Do note that SSL setup steps may differ depending on several factors, such as your chosen SSL provider, proxy server used if any (e.g.: Apache HTTPD), operating system, etc.