if you have previously installed grub-customizer uninstall it by typing:
sudo dnf remove grub-customizer
then type the following to install a new version that works on fedora 28 64bit:
sudo dnf install https://www.rpmfind.net/linux/fedora/linux/updates/28/Everything/x86_64/Packages/g/grub-customizer-5.0.8-1.fc28.x86_64.rpm
If installing on fedora 29 64bit use this url instead: https://www.rpmfind.net/linux/fedora/linux/development/rawhide/Everything/x86_64/os/Packages/g/grub-customizer-5.0.8-2.fc29.x86_64.rpm
enter your password and click "y" when it asks you to install it.
Older versions of grub customizer will not open in fedora 28 even if you log out and change the session to x.org instead of the default wayland. This version should install correctly without errors and won't require you to change from wayland the x.org to open it.