title: Yup: How to validate field only when it exists?
body: <p>Is it possible to validate a field only when it exists?</p>
  <p>I'm going to create an application. A field(email) may / may not display on step 2, it depends on the step 1's selected option. The problem is that I can't put <code>email: Yup.string().Required()</code>, it causes that the validation run all the time even the field is not displayed.</p>