How can I make sure vim does not open files and other buffers on NerdTree window?

" If more than one window and previous buffer was NERDTree, go back to it.
autocmd BufEnter * if bufname('#') =~# "^NERD_tree_" && winnr('$') > 1 | b# | endif

If you are using vim-plug, you'll also need to add these lines to avoid crashes when calling vim-plug functions while the cursor is on the NERDTree window:

let g:plug_window = 'noautocmd vertical topleft new'