I saw this issue the other day on the Frndly thread Frndly TV for Channels - #444 by fortuna15, and I think it may have been mentioned again on the How do I install Docker on my Mac to use with Pluto - #5 by jabski thread. I've done this by accident a number of time too.
When entering in the URLs on the Custom Channels form, accidentally including spaces before the URL will give this error.
invalid source url: parse " http://whatever_ip:8183/playlist.m3u8": first path segment in URL cannot contain colon
I understand this is very minor and it really comes down to an end-user mistake. Its not a bug, nor a feature request. Maybe consider it a cosmetic suggestion. But URL validation to trim out the white space is something to possibly keep in mind. I know the devs have been doing an awesome job tackling all sorts of far more important issues. I just wanted to note a little thing. It may be helpful for people new to setting things up who may be more prone to making that mistake. Who will see that error and misguidedly go hunting for rouge colons.