To get rid of the pop up message go to below location and click on Accept non-trusted certificates automatically.

File | Settings | Tools | Server Certificates for Windows and Linux 
IntelliJ IDEA | Preferences | Tools | Server Certificates for macOS       

參考： https://intellij-support.jetbrains.com/hc/en-us/community/posts/115000131364-Server-s-Certificate-is-not-trusted-pop-up