Question on SLoad SSave and NERDTree

[lesar]

I'm new to vim-startify and I like it a lot.
I'm using NERDTree too.
If I save session and NERDTree is open I have trouble on restore session so I find this workaround:

let g:startify_session_before_save = [
            \ 'silent! NERDTreeClose'
            \ ]
autocmd SessionLoadpost * NERDTree

Now I can save session and restore showing NERDTree without trouble.
Is there a better way?
Can't I save NERDTree inside session by my poor configuration?
Other can save NERDTree inside session without problem?

best regards, Leonardo