I very new to emacs and I installed evil-mode and haskell-mode.
The problem that I have is that every time I press 'o' an haskell-mode-hook window pops up and tells me that the variable is nil and I should configure it.
It only appears when I didn't specify an indention mode, but all those indention modes are super strange and I don't want them to be active.
Is there a way that I can prevent those 'pop ups'?
IIUC, this is a misfeature of the new Haskell-mode. You should contact the maintainers about it.
The back story is that Haskell-mode accumulated over the years many new features that were added to haskell-mode-hook by default, which was a bad idea. So, the new maintainer is apparently trying to fix the problem by setting haskell-mode-hook to nil by default (as god intended) but then remind users that this is probably not what they want, since most users will want to use some haskell indentation package (they may not be 100% ideal all the time, because they can't read your mind, but at least the haskell-simple-indent should be sufficiently simple not to feel "super strange").