undefined local variable or method 'main_page' - ruby-on-rails

Question

In my application.html.erb I have a menu, which is displayed on every page. But I can't use 'sign out' link, an error appears.
undefined local variable or method 'main_page'
I have 'main_page' in my routes and I don't understand why it's not working.
I've tried to use 'sign out' link from my show view in the User controller's action, and it worked fine.
show.html
<% content_for :user_form do %>
<p id="notice"><%= notice %></p>
<p>
<strong>Name:</strong>
<%= #user.name %>
</p>
<p>
<strong>Email:</strong>
<%= #user.email %>
</p>
<%= link_to 'Edit', edit_user_path(#user) %>
<%= link_to 'My dates', :controller => :calendar, :action => :month_for_user %>
<%= link_to 'sign out', :controller => :sessions, :action => :destroy %>
<%end%>
Why there is an error when the link is used from application.html? Although 'My dates' link works. How can I fix it?
application.html
<div id = 'user_menu' >
<p id="notice"><%= notice %></p>
<p>
<strong>Name:</strong>
<%= current_user.name %>
</p>
<p>
<strong>Email:</strong>
<%= current_user.email %>
</p>
<%= link_to 'Edit', edit_user_path(current_user) %>
<%= link_to 'My dates', :controller => 'calendar', :action => 'month_for_user' %>
<%= link_to 'sign out', :controller => 'sessions', :action => 'destroy' %>
<% end %>
</div>
sessions_controller
def destroy
redirect_to main_page
sign_out
end
routes
Prefix Verb URI Pattern Controller#Action
meetings GET /meetings(.:format) meetings#index
POST /meetings(.:format) meetings#create
new_meeting GET /meetings/new(.:format) meetings#new
edit_meeting GET /meetings/:id/edit(.:format) meetings#edit
meeting GET /meetings/:id(.:format) meetings#show
PATCH /meetings/:id(.:format) meetings#update
PUT /meetings/:id(.:format) meetings#update
DELETE /meetings/:id(.:format) meetings#destroy
new_user GET /users/new(.:format) users#new
users GET /users(.:format) users#index
main_page GET /main_page(.:format) welcome#domain
new_session GET /sign_in(.:format) sessions#new
signout GET /sign_out(.:format) sessions#destroy
sessions POST /sessions(.:format) sessions#create
GET /sessions/new(.:format) sessions#new
edit_session GET /sessions/:id/edit(.:format) sessions#edit
session PATCH /sessions/:id(.:format) sessions#update
PUT /sessions/:id(.:format) sessions#update
DELETE /sessions/:id(.:format) sessions#destroy
GET /sessions/:id(.:format) welcome#domain
GET /users(.:format) users#index
POST /users(.:format) users#create
GET /users/new(.:format) users#new
edit_user GET /users/:id/edit(.:format) users#edit
user GET /users/:id(.:format) users#show
PATCH /users/:id(.:format) users#update
PUT /users/:id(.:format) users#update
DELETE /users/:id(.:format) users#destroy
month_for_user GET /month_for_user(.:format) calendar#month_for_user
Rails is new for me, and maybe it's a silly question, but I honestly tried to find an answer unfortunately without results. Please, help me to fix this problem! Thank you

Answer 1

undefined local variable or method 'main_page'
The error comes from the destroy method in your sessions_controller. It should be
def destroy
redirect_to main_page_url #or action: "main_page"
sign_out
end