Vagrantfile.rb

# Here are the lines in the "Parent file" that the error refers to:
# Allow an untracked Vagrantfile to modify the configurations
  #eval File.read "#{host_config_dir}/Vagrantfile.local" if File.exist?("#{host_config_dir}/Vagrantfile.local")
  if File.exist?("#{host_config_dir}/Vagrantfile.local")
    eval File.read "#{host_config_dir}/Vagrantfile.local" # This is line 191
  end
  
# Here are the primary lines from the error:
#/opt/drupalvm/master-env/Vagrantfile:191:in `eval': undefined method `[]=' for nil:NilClass (NoMethodError)
	#from (eval):1:in `each'
	#from (eval):1:in `block in <top (required)>'
	#from /opt/drupalvm/master-env/Vagrantfile:191:in `eval'
	#from /opt/drupalvm/master-env/Vagrantfile:191:in `block in <top (required)>'
	#from /opt/vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/config/v2/loader.rb:37:in `call'

# This one works.
vconfig['vagrant_synced_folders'].each do |synced_folder|
  case synced_folder['type']
  when "nfs"
    guest_path = synced_folder['destination']
    host_path = File.expand_path(synced_folder['local_path'])
    config.vm.synced_folders[guest_path][:guestpath] = "/var/nfs#{host_path}"
    config.bindfs.bind_folder "/var/nfs#{host_path}", guest_path,
      u: 'vagrant',
      g: 'www-data',
      perms: 'u=rwX:g=rwD',
      o: 'nonempty'
    config.nfs.map_uid = Process.uid
    config.nfs.map_gid = Process.gid
  end

# This one doesn't.
  case synced_folder['files']
  when "/sites/default/files"
    host_files = File.expand_path(synced_folder['local_path'] + synced_folder['files'])
    guest_files = synced_folder['destination'] + synced_folder['files']
    nfs_path = "/var/nfs/#{synced_folder['id']}/#{host_files}"
    config.vm.synced_folders[guest_files][:guest_files] = nfs_path
    config.bindfs.bind_folder host_files, guest_files,
      u: 'www-data',
      g: 'www-data',
      perms: 'u=rwX:g=rwX',
      o: 'nonempty'
    config.nfs.map_uid = Process.uid
    config.nfs.map_gid = Process.gid
  end
end