Icon doesn't show up for a custom app

[rado84-github]

This extension has a huge potential! I'm using it for like 10 minutes and I already love it! I added SMPlayer and it works! :D
There's one tiny problem about it (tiny bc it DOESN'T affect how the extension works) - after I chose desktop file, I also added a custom icon (150x150 png file with transparent background) but when I click the extension icon on the toolbar, it reads "smplayer" but the icon for it is missing. There's only a small transparent square in its place.
The icon itself is located on an NTFS partition with full read and write permissions - all written in fstab, so the extension shouldn't have a problem with accessing it.