oidc-client CheckSessionIFrame fires properly one time, then fails ever interval thereafter
<p>This may not actually be an issue with Identity Server or the oidc-client, but I am having trouble pinning down the problem.  I am running this through System.js in an Aurelia application, so it's possible the issue originates from one of these external libraries.</p>

<p>In <code>CheckSessionIFrame.start(session_state)</code>, we have the following code:</p>

<pre><code>this._timer = window.setInterval(() =&gt; {
  this._frame.contentWindow.postMessage(this._client_id + " " + this._session_state, this._frame_origin);
}, this._interval);
</code></pre>

<p>The first time the interval fires, there appear to be no problems.  The iFrame's contentWindow exists (as expected) and the postMessage method is called without issue.  Two seconds later, when the interval fires again, <code>this._frame.contentWindow</code> is undefined - so my best guess is the iFrame is dying somehow.  Again, this may not be an issue with oidc-client, but I'm looking for any helpful guidance on what could cause this iFrame to die (perhaps internally it could be dying on a script?) such as a missing necessary config value for oidc-client.</p>