Block acknowledgement submit when no category is available

app/views/admin/ask_acknowledgements/_form.html.erb

@@ -3,6 +3,17 @@
     <%= f.label :content, t('category'), class: 'control-label' %>
     <%= f.select :category_id, options_for_select(@categories) %>
   </div>
+  <script type="text/javascript">
+    $(function(){
+      var $cat = $("select[name='<%= f.object_name %>[category_id]']");
+      $cat.closest("form").off("submit.category_check").on("submit.category_check", function(){
+        if(!$cat.val()){
+          alert("<%= j((I18n.locale.to_s == 'zh_tw') ? '沒有可選擇的類別。每個類別只能設定一筆，若尚未建立類別或所有類別都已設定，請先到「類別」頁面新增類別，或直接編輯既有的設定。' : 'No category available. Each category can only have one setting. If no category exists or all categories are already set, please add a new category on the Categories page first, or edit an existing setting.') %>");
+          return false;
+        }
+      });
+    });
+  </script>
 <% end %>
 <div class="control-group">
   <%= f.label :content, t('ask.acknowledgements'), class: 'control-label' %>
@@ -13,4 +24,4 @@
   <%= f.submit t('submit'), class: 'btn btn-primary' %>
   <%= f.button t('cancel'), type: 'button', class: 'btn', onclick: 'history.back();' %>
 </div>
- 
+