Remove instance variables from helper usage (#24203)

app/controllers/application_controller.rb

@@ -19,6 +19,7 @@ class ApplicationController < ActionController::Base
   helper_method :omniauth_only?
   helper_method :sso_account_settings
   helper_method :whitelist_mode?
+  helper_method :body_class_string
 
   rescue_from ActionController::ParameterMissing, Paperclip::AdapterRegistry::NoHandlerError, with: :bad_request
   rescue_from Mastodon::NotPermittedError, with: :forbidden
@@ -148,6 +149,10 @@ class ApplicationController < ActionController::Base
     current_user.setting_theme
   end
 
+  def body_class_string
+    @body_classes || ''
+  end
+
   def respond_with_error(code)
     respond_to do |format|
       format.any  { render "errors/#{code}", layout: 'error', status: code, formats: [:html] }