Panics when there are no urls in url set

[meirf]

I expected the below code to return a sitemap with headings but no urls. Instead the code panics.

package main
import (
	"github.com/ikeikeikeike/go-sitemap-generator/stm"
)
func main() {
	sm := stm.NewSitemap()
	sm.Create()
        sm.XMLContent()
}