mailer previews for `NullMail` instances. Closes #19849.

3 files changed, 39 insertions, 6 deletions

diff --git a/railties/lib/rails/mailers_controller.rb b/railties/lib/rails/mailers_controller.rb
index 32740d66da..cd1c097eb6 100644
--- a/railties/lib/rails/mailers_controller.rb
+++ b/railties/lib/rails/mailers_controller.rb
@@ -16,10 +16,10 @@ class Rails::MailersController < Rails::ApplicationController # :nodoc:
       @page_title = "Mailer Previews for #{@preview.preview_name}"
       render action: 'mailer'
     else
-      email = File.basename(params[:path])
+      @email_action = File.basename(params[:path])
 
-      if @preview.email_exists?(email)
-        @email = @preview.call(email)
+      if @preview.email_exists?(@email_action)
+        @email = @preview.call(@email_action)
 
         if params[:part]
           part_type = Mime::Type.lookup(params[:part])
@@ -28,14 +28,14 @@ class Rails::MailersController < Rails::ApplicationController # :nodoc:
             response.content_type = part_type
             render text: part.respond_to?(:decoded) ? part.decoded : part
           else
-            raise AbstractController::ActionNotFound, "Email part '#{part_type}' not found in #{@preview.name}##{email}"
+            raise AbstractController::ActionNotFound, "Email part '#{part_type}' not found in #{@preview.name}##{@email_action}"
           end
         else
           @part = find_preferred_part(request.format, Mime::HTML, Mime::TEXT)
           render action: 'email', layout: false, formats: %w[html]
         end
       else
-        raise AbstractController::ActionNotFound, "Email '#{email}' not found in #{@preview.name}"
+        raise AbstractController::ActionNotFound, "Email '#{@email_action}' not found in #{@preview.name}"
       end
     end
   end
diff --git a/railties/lib/rails/templates/rails/mailers/email.html.erb b/railties/lib/rails/templates/rails/mailers/email.html.erb
index cce818d103..afca6368d5 100644
--- a/railties/lib/rails/templates/rails/mailers/email.html.erb
+++ b/railties/lib/rails/templates/rails/mailers/email.html.erb
@@ -103,7 +103,14 @@
   </dl>
 </header>
 
-<iframe seamless name="messageBody" src="?part=<%= Rack::Utils.escape(@part.mime_type) %>"></iframe>
+<% if @part.mime_type %>
+  <iframe seamless name="messageBody" src="?part=<%= Rack::Utils.escape(@part.mime_type) %>"></iframe>
+<% else %>
+  <p>
+    You are trying to preview an email that does not have any content.
+    This is probably because the <em>mail</em> method has not been called in <em><%= @preview.preview_name %>#<%= @email_action %></em>.
+  </p>
+<% end %>
 
 </body>
 </html>
diff --git a/railties/test/application/mailer_previews_test.rb b/railties/test/application/mailer_previews_test.rb
index 1752a9f3c6..83501a7f11 100644
--- a/railties/test/application/mailer_previews_test.rb
+++ b/railties/test/application/mailer_previews_test.rb
@@ -327,6 +327,32 @@ module ApplicationTests
       assert_match "Email &#39;bar&#39; not found in NotifierPreview", last_response.body
     end
 
+    test "mailer preview NullMail" do
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            # does not call +mail+
+          end
+        end
+      RUBY
+
+      mailer_preview 'notifier', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app('development')
+
+      get "/rails/mailers/notifier/foo"
+      assert_match "You are trying to preview an email that does not have any content.", last_response.body
+      assert_match "notifier#foo", last_response.body
+    end
+
     test "mailer preview email part not found" do
       mailer 'notifier', <<-RUBY
         class Notifier < ActionMailer::Base