Fix popover model responder chain crash

Just a minor update to fix this error:

[Popovers] - No PopoverModel present in responder chain ((self)) - has the source view been installed into a window? Please file a bug report (https://github.com/aheze/Popovers/issues).

I still wasn't able to get rid of this entirely, but for now I just made Popovers create a new PopoverModel.