title: how to set custom return type nullable in kotlin
body: <p>I made a personal data type &quot;email&quot;
  how can I make this type nullable in value.</p>
  <p>For example.</p>
  <pre><code>private lateinit var a: email?
  </code></pre>
  <p>or</p>
  <pre><code>fun makeEmailAddress(a: String): email? {
     // TODO
  }
  </code></pre>