title: How to give alert message when InputText is empty and submitted?
body: <p>I use <code>react native</code> and <code>TextInput</code>.</p>
  <p>And if user submits inputText with empty value, I want to give them a message 'please write a comment'.
  So I make a <code>register</code> module from <code>useForm</code> with <code>required</code> function.</p>
  <p>It seems it prevents empty value created correctly, but alert message is not shown.</p>
  <pre><code>      &lt;WritingText
          {...register(&quot;comments&quot;, { required: &quot;댓글을 적어주세요.&quot; })}
          placeholder=&quot;댓글을 입력하세요&quot;
          onChangeText={(text) =&gt; setValue(&quot;comments&quot;, text)}
          onSubmitEditing={handleSubmit(onValid)}
        &gt;&lt;/WritingText&gt;
  </code></pre>