Nested formed by cocoon cannot be saved - ruby

Question

Problem
I'm coding user's profile page on Rails.
I added a gem "language-select" for users to choose a language which they're learning. And users might learn several languages, so I added gem "cocoon" as well.
The first language which users choose is successfully saved, but others which were chosen by cocoon cannot be saved. I would like them to be saved and being showed on their profile page.
How I reached this problem
I added cocoon by refering this page and code.README in github
However, "Save" button didn't react anything, so I made the form_for's method "post".
<%= form_for #user, method: :post do |f| %>
<div id='languages'>
<%= f.fields_for :languages do |language| %>
<%= render 'language_fields', :f => language %>
<% end %>
<div class='links'>
<%= link_to_add_association 'add language', f, :languages %>
</div>
</div>
<% end %>
Then "Save" button worked, but an error occured.
ActionController::InvalidAuthenticityToken in UsersController#update
ActionController::InvalidAuthenticityToken
Extracted source (around line #211):
def handle_unverified_request
raise ActionController::InvalidAuthenticityToken
end
end
end
Therefore, I added "protect_from_forgery with: :null_session" to application_controller.rb. Then the error disappeared, but the problem which I mentioned above occered. The first language which users choose is saved, but other ones which were chosen by cocoon cannot be saved.
（users#show.html.erb）
<h2><%= #user.language %></h2>
（users#edit.html.erb）
<%= form_tag("/users/#{#user.id}/update", {multipart: true}) do %>
<table>
<p>Languages</p>
<%= select_tag(:language,options_for_select(languages)) %>
<%= render 'users/form' %>
<input type="submit" value="Submit">
</table>
<% end %>
（users#_form.html.erb）
Partial of cocoon.
<%= form_for #user, method: :post do |f| %>
<div id='languages'>
<%= f.fields_for :languages do |language| %>
<%= render 'language_fields', :f => language %>
<% end %>
<div class='links'>
<%= link_to_add_association 'add language', f, :languages %>
</div>
</div>
<% end %>
（users#_language.html.erb）
Partial of cocoon.
<div class='nested-fields'>
<div class="field">
<%= select_tag(:language,options_for_select(languages)) %>
</div>
<%= link_to_remove_association "remove language", f %>
</div>
（users_controller.rb）
Even though I settled "redirect_to("/users/#{#user.id}")" here, it redirects to "/posts/index" after press "Save" button. And the sentence "You're already logged in" which I settled at "forbid_login_user" in "application_controller.rb" is shown too.
before_action :authenticate_user, {only: [:index, :show, :edit, :update]}
before_action :forbid_login_user, {only: [:new, :create, :login_form, :login]}
before_action :ensure_correct_user, {only: [:edit, :update]}
def edit
#user = User.find_by(id: params[:id])
end
def user_params
params.require(:user).permit(:name, :description, languages_attributes: [:id, :description, :done, :_destroy])
end
def update
#user = User.find_by(id: params[:id])
#user.language = params[:language]
if params[:image]
#user.image_name = "#{#user.id}.jpg"
image = params[:image]
File.binwrite("public/user_images/#{#user.image_name}", image.read)
end
if params[:cover_image]
#user.cover_image_name = "#{#user.id}_cover.jpg"
cover_image = params[:cover_image]
File.binwrite("public/user_cover_images/#{#user.cover_image_name}", cover_image.read)
end
if #user.save
flash[:notice] = "Edited user's information"
redirect_to("/users/#{#user.id}")
else
render("users/edit")
end
end
（application_controller.rb）
class ApplicationController < ActionController::Base
protect_from_forgery with: :null_session
before_action :set_current_user
def set_current_user
#current_user = User.find_by(id: session[:user_id])
end
def authenticate_user
if #current_user == nil
flash[:notice] = "You need to log in"
redirect_to("/login")
end
end
def forbid_login_user
if #current_user
flash[:notice] = "You're already logged in"
redirect_to("/posts/index")
end
end
end
I will add more code if it needs to be refered. Thank you very much.
Version
ruby 2.6.4p104 / RubyGems 3.0.3 / Rails 5.2.3
Postscript
When I choose only one language, it can be saved.
When I choose like several languages like below, the problem which I mentoined above happens. Nothing is saved.
But after putting "skip_before_action :verify_authenticity_token" in application_controller.rb, only "English"(the last one) is saved (checked by rails console).
"Japanese" (chose by original form)
"English" (added and chose by cocoon)
Anyway, only 1 language can be saved so far.