Navigator loses Frame reference when displaying multiple Frames on IOS

[Lehren]

The navigator will break navigation on IOS when a new Frame instance is created somewhere. For instance when showing a modal that is structured like so:

<Frame>
    <Page>
        <Label> Hello </Label>
    </Page>
</Frame

And then closing the modal, all navigation stops, at least on IOS. My guess is due to the fact the navigator cannot find the 'default' Frame anymore, or it is using the frame of the closed and discarded modal. Resulting in errors.

[Lehren]

The fix seems to be to set the id and ref of the modal Frame to something other than 'default'. Might be useful to put into the docs or something. Or for any future visitors :)

[rigor789]

Tracked at #27