 <p>"Exception: The null object does not have a method 'querySelector'."</p> <p>Could that error possibly be because the <code>&lt;polymer-element&gt;</code> definition has no <code>&lt;template&gt;</code>? Perhaps a shadow DOM is never created in that instance, so <code>shadowRoot</code> is <code>null</code>. Just guessing here...</p> <p>I wonder, also, if <code>this..append()</code> attaches elements to the shadow DOM. Seems unlikely.</p>