FIX: Missing template error when rendering `topics#show` error message.

app/controllers/application_controller.rb

@@ -162,7 +162,7 @@ class ApplicationController < ActionController::Base
     if show_json_errors
       # HACK: do not use render_json_error for topics#show
       if request.params[:controller] == 'topics' && request.params[:action] == 'show'
-        return render status: status_code, layout: false, text: (status_code == 404 || status_code == 410) ? build_not_found_page(status_code) : I18n.t(type)
+        return render status: status_code, layout: false, plain: (status_code == 404 || status_code == 410) ? build_not_found_page(status_code) : I18n.t(type)
       end
 
       render_json_error I18n.t(opts[:custom_message] || type), type: type, status: status_code

spec/requests/topics_controller_spec.rb

@@ -4,6 +4,21 @@ RSpec.describe TopicsController do
   let(:topic) { Fabricate(:topic) }
   let(:user) { Fabricate(:user) }
 
+  describe '#show' do
+    let(:private_topic) { Fabricate(:private_message_topic) }
+
+    describe 'when topic is not allowed' do
+      it 'should return the right response' do
+        sign_in(user)
+
+        get "/t/#{private_topic.id}.json"
+
+        expect(response.status).to eq(403)
+        expect(response.body).to eq(I18n.t('invalid_access'))
+      end
+    end
+  end
+
   describe '#timings' do
     let(:post_1) { Fabricate(:post, topic: topic) }