After (in this PR) introducing `_openDeferred = null` to always cleanup after calls `open()`, this was throwing an error when editing a JS file (in-app and in a unit test failure) because that use case is just a call to `reconnect`. Reconnecting just unloads/reloads agents and in that case `_openDeferred` will be null. 

I moved this code into `_onInterstitialPageLoad` instead since it's only ever called during the initial `open()`.