How to ignore nerdtree buffer in vimrc settings?

I have the following line in my init.vim:

if !&readonly
  set fileencoding=utf-8
endif

Which sets the fileencoding to utf-8. It works fine, unless I have my cursor in a nerdtree window, in which case I get the following error:

I guess that it's trying to set the nerdtree window to utf-8. I think that I could solve this by checking for nerdtree in the if statement (so that it doesn't try to set fileencoding for nerdtree windows). How do I do that?

I've checked and the nerdtree window is : noreadonly & nomodifiable. The message which you've received tells you all. You cannot modify a window which is 'nomodifiable' even by setting an option on it. Try:

if !&readonly && &modifiable
  set fileencoding=utf-8
endif

Other option is to remember the current state in some variable:

if !&readonly
    let prev_modifiable = &modifiable
    set modifiable
    set fileencoding=utf-8
    let &modifiable=prev_modifiable
endif