There is a nil pointer exception because the code assigns a nil value to an interface (e.Listener) which ends up creating a non-nil interface containing a nil pointer. Using this will result in the nil pointer exception and checking the e.Listener against nil will be false -- which is why the Listener is not recreated and overwritten the during the second call to Start.

Small code example that triggers this error (given an existing echo router):

if err := e.Start(":6060"); err != nil {
	// retry with random port
	if r := e.Start(":"); r != nil {
		log.Fatal(r)
	}
}