Unknown error on the theme files

[mDonchev]

Hello, 

As you can see from the attached image there is an error somewhere on my theme (took from the Debut theme) which is not showing me where the error is.
If I make an error on purpose in some of my files, the error message says exactly where the error is, but otherwise it shows nothing like you see.
I have linter and the theme CLI is not showing anything to have a problem, however the error still exists.

How can I see where the error is so I can fix that?

Thanks

p.s. This message disappears when I switch to customize other pages of the theme.