Error：(webloop:3579): Gtk-WARNING **: cannot open display:

[touei]

OS Version：CentOS-7-x86_64-Minimal-1611

WebLoop has been installed successfully.
When I run my own program. I get the following error message.

(webloop:3579): Gtk-WARNING **: cannot open display: 

[mvz]

From what I know of Gtk+, Webloop is probably only headless in the sense that it doesn't open a window. It still needs an X server.