title: Material-UI date pickers' Icons are not visible
body: <p><a href="https://i.stack.imgur.com/0UgKq.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0UgKq.png" alt="enter image description here"></a>I am using material-ui for Datepicker component <code>import Grid from "@material-ui/core/Grid";</code> date picker icons are not showing. </p>
  
  <p>Please find codesandbox link here: <a href="https://codesandbox.io/s/material-demo-7vzo5" rel="nofollow noreferrer">https://codesandbox.io/s/material-demo-7vzo5</a>.</p>