title: how can I clear Native-base Input text?
body: <p>I try this</p>
  
  <pre><code>  &lt;Input placeholder="search...."
           ref={(ref) =&gt; { this.SearchInput = ref; }}
           /&gt;
    &lt;Button transparent onPress={this.clear}&gt;
        &lt;Ionicons name="ios-close" /&gt;
    &lt;/Button&gt;
  </code></pre>
  
  <p>and function:</p>
  
  <pre><code>  clear = () =&gt; {
      this.SearchInput.clear();
    }
  </code></pre>
  
  <p>I get this error:</p>
  
  <blockquote>
    <p>this.SearchInput.clear() is not a function</p>
  </blockquote>