CentOS 8 执行firewall-config显示cannot open display

在虚拟机中的CentOS 8中安装完firewall-config之后，执行firewall-config
显示cannot open display。

[root@localhost yum.repos.d]# firewall-config
Unable to init server: Could not connect: Connection refused
Unable to init server: Could not connect: Connection refused
Unable to init server: Could not connect: Connection refused

(firewall-config:7284): Gtk-WARNING **: 01:26:49.499: cannot open display:
[root@localhost yum.repos.d]#

按照网上方法提示，运行 xhost +等命令，但是无效。

[root@localhost ~]# xhost +
No protocol specified
xhost:  unable to open display ":0.0"

重启虚拟机后问题解决。