document.getElementById returns null on Chrome

[marcmmx]

The onclick handler calls the clearInput() function below. But it seems that the getElementById doesn't work on Chrome in Polymer. No problems with Firefox and Safari.
function clearInput() {
document.getElementById('inputWithButton').value='';
}

Uncaught TypeError: Cannot set property 'value' of null

[aeon3k]

same problem with chrome Version 54.0.2840.99 m on windows 7 32 bits
No problem with Firefox.

<paper-button raised class="custom indigo" onclick="document.getElementById('FormID').submit();"> <iron-icon icon="send"></iron-icon> Submit </paper-button>
Uncaught TypeError: Cannot read property 'submit' of null(…)