paper-input-container attach causes "Cannot read property 'addEventListener' of null"

[stfnzl]

First of all, thanks for this great library!

I'm trying to run a simple example using paper-input-container like in the example below:

<paper-input-container>
  <div slot="input">
    <input type="text" />
  </div>
</paper-input-container>

Looking at the console I get the following error:

Uncaught TypeError: Cannot read property 'addEventListener' of null
    at HTMLElement.attached (paper-input-container.js:534)
    at HTMLElement.attached (class.js:391)
    at HTMLElement.connectedCallback (legacy-element-mixin.js:228)
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke (zone.js:391)
    at Zone.push../node_modules/zone.js/dist/zone.js.Zone.runGuarded (zone.js:161)
    at HTMLElement.connectedCallback (zone.js:144)
    at EmulatedEncapsulationDomRenderer2.push../node_modules/@angular/platform-browser/fesm5/platform-browser.js.DefaultDomRenderer2.appendChild (platform-browser.js:1053)
    at DebugRenderer2.push../node_modules/@angular/core/fesm5/core.js.DebugRenderer2.appendChild (core.js:11429)
    at createElement (core.js:8125)
    at createViewNodes (core.js:10360)

Setting a breakpoint at paper-input-container.js:534 I see that the null object is _inputElement (get by dom(this).querySelector(this._inputSelector)).

Inspecting the dom with the breakpoint active I can see that the input is not jet present in the DOM.

How could we defer the paper-input-container attached until the input has need dumped into the DOM?

[stfnzl]

Using <router-outlet></router-outlet> everything works as expected.
Feel free to close the issue if you do not have any comment on that :)