App.prototype._finishInit should handle unexpected null

App.prototype._finishInit should always emit the error even if `script` is undefined.

---
This idiom is similar to the accessor variant of coffeescript's existential operator
```coffeescript
json = script?.nextSibling?.innerHTML
```
but any falsey value along the chain will produce and end result of `undefined` while the coffeescript version explicity checks `null` and `undefined` instead of all falsey values.

^ for my own reference, folks.