### Code to prevent this error:
```
polymer-fn.js:43 Uncaught (in promise) DOMException: Failed to execute 'define' on 'CustomElementRegistry': this name has already been used with this registry
at Polymer
```
#### You just have to add before this line:
```
window.customElements.define('my-app', MyApp);
```
#### This is the code that will solve the problem:
```
const _customElementsDefine = window.customElements.define;
window.customElements.define = function(name, clazz, config) {
if (!customElements.get(name)) {
_customElementsDefine.call(window.customElements, name, clazz, config);
}
};
```
#### This is necessary to import the paper-dropdown-menu
```
import '@polymer/paper-item/paper-item.js';
import '@polymer/paper-listbox/paper-listbox.js';
import '@polymer/paper-dropdown-menu/paper-dropdown-menu.js';
```