Unsupported protocol scheme ""

Hello ,

I am just starting with drone .
I am try to get the slack notification when every my build is success or fail

so i wrote this xml file
steps:

this example were taken from this tutorial :- http://plugins.drone.io/drone-plugins/drone-slack/

but whenever i try to run the build
it show me the error which i have mention in my subject

how to solve this error

regards
Harsh