必需属性在选择中不起作用

Question

在单击按钮后，必选属性不起作用。 我有三个按钮btn-insert，btn-update btn-insertvideo，但是按btn-insert后，它不生效

我的代码：

<form method="POST" id="topic_form" class="form-horizontal well">
    <div class="col-sm-12">
          <select  id="mainCatItem" name="mainCatItem" class="form-control" required>
            <option value="">Select Category</option>}
            option
            <?php foreach($fetchcat as $fetchcategory): ?>
                <option value="<?php echo $fetchcategory->main_category_id; ?>"><?php echo $fetchcategory->main_category_name; ?></option>
            <?php endforeach; ?>
        </select>
     </div>

      <div class="form-group row mb-0">
       <div class="col-sm-6">
          <Button type="submit" id="btn-insert" value="Submit" class="btn btn-primary">Add</Button>
          <button type="submit" id="btn-update" value="Submit" class="btn btn-primary">Update</button>
       </div>
    </div>
    <input id="btn-insertvideo" type="button" value="Submit" onclick="save()" class="btn btn-primary">
</form>
<script>
    $('#btn-insert').click(function(e){
     e.preventDefault();
      $.ajax({
            url: "<?php echo base_url();?>admin_controller/WebsiteContentController/createtopic",
            type: "POST",
            data: $('#topic_form').serialize(),
            dataType:'text',
            success: function(response)
            {
              alert(response);
            }
    });
  });
  </script>

点击btn-insert后，我想显示要求消息 如果未选择dopdown，我想在btn-insert单击后停止该ajax