title: How to InputRef works in material ui core 3.9.2
body: <p>In <b>Material ui core 3.9.2</b><br/>
  On inputRef={input => {
                this.input = input;
              }}<br/>
  <br/>
  <b>Error Shows</b> <br/>
  <br/>
  TypeError: Cannot set property 'input' of undefined</p>
  
  <p>If we use this.email instead of this.input <br>
  <br/>
  <b>Then Error Shows</b> <br/>
  <br/>
  TypeError: Cannot set property 'email' of undefined</p>
  
  <p><b>This is TextField Code </b></p>
  
  <pre><code>&lt;TextField
              id="login-email"
              label="Email/MobileNo"
              required
              fullWidth
              type="email"
              className={classes.textField}
              inputRef={el =&gt; {
                this.input = el;
              }}
              or
              inputRef={el =&gt; this.email = el;}
              margin="normal"
          /&gt;
  </code></pre>