Prevent invalid newsletters from being sent

Why:

Newsletters without a valid user segment can't be sent since there is no
recipient user email list.

How:

* Hiding the send button at the form
* Preventing an invalid newsletter from being delivered at the controller

app/controllers/admin/newsletters_controller.rb

@@ -46,11 +46,16 @@ class Admin::NewslettersController < Admin::BaseController
 
   def deliver
     @newsletter = Newsletter.find(params[:id])
-    Mailer.newsletter(@newsletter).deliver_later
 
-    @newsletter.update(sent_at: Time.current)
+    if @newsletter.valid?
+      Mailer.newsletter(@newsletter).deliver_later
+      @newsletter.update(sent_at: Time.current)
+      flash[:notice] = t("admin.newsletters.send_success")
+    else
+      flash[:error] = t("admin.segment_recipient.invalid_recipients_segment")
+    end
 
-    redirect_to [:admin, @newsletter], notice: t("admin.newsletters.send_success")
+    redirect_to [:admin, @newsletter]
   end
 
   private

app/views/admin/newsletters/show.html.erb

@@ -42,8 +42,9 @@
   </div>
 </div>
 
-<% if @newsletter.draft? %>
-  <%= link_to t("admin.newsletters.show.send"), deliver_admin_newsletter_path(@newsletter),
+<% if @newsletter.draft? && @newsletter.valid_segment_recipient? %>
+  <%= link_to t("admin.newsletters.show.send"),
+              deliver_admin_newsletter_path(@newsletter),
               "data-alert": t("admin.newsletters.show.send_alert", n: recipients_count),
               method: :post,
               id: "js-send-newsletter-alert",