title: How to disable the SaveButton when the SimpleForm is invalid in a react-admin app?
body: <p>In a react-admin <a href="https://marmelab.com/react-admin/CreateEdit.html#the-simpleform-component" rel="nofollow noreferrer">SimpleForm</a> component validation is working fine when I click the save button. The field that is required is highlighted and marked red when I click the save button.</p>
  
  <p>I'd like to add a className to the <code>SaveButton</code> as long as the form is invalid. This way I can make it clear to the user that he's not done with the form yet and prevent the user from clicking it.</p>
  
  <p>This is a simplified version of such a SimpleForm.</p>
  
  <pre><code>import {
      required,
      //...
  } from 'react-admin';
  
  const UserCreateToolbar = props =&gt;
      &lt;Toolbar {...props}&gt;
          &lt;SaveButton
              label="user.action.save_and_show"
              redirect="show"
              submitOnEnter={true}
          /&gt;
      &lt;/Toolbar&gt;;
  
  export const UserCreate = props =&gt;
      &lt;Create {...props}&gt;
          &lt;SimpleForm
              toolbar={&lt;UserCreateToolbar /&gt;}
          &gt;
              &lt;TextInput source="name" validate={[required()]} /&gt;
          &lt;/SimpleForm&gt;
      &lt;/Create&gt;;
  </code></pre>