Delay loading of percentageFromCfi until displayedCFI is ready.

[Neulus]

When loading a .txt file, I observed that percentageFromCfi is invoked with displayedCFI before displayedCFI is fully initialized. This premature loading can cause issues similar to those reported in #47.

Changes

• Added a check to ensure displayedCFI is loaded before invoking percentageFromCfi.

[btpf]

Nice job tracking it down.

I think a small change I would prefer is just setting the currentPercentage to 0 if the displayedCFI is not ready.

24a6b4d#diff-5fbaa34ea6def579f01b427ae5834cbdd8811adc65b5fa7cd82520e075eabb7eR110-R114
to just

  // wait until displayedCFI is loaded
  const currentPercentage = displayedCFI ? renditionInstance?.book.locations.percentageFromCfi(displayedCFI):0

Ill merge this in and make the changes. Thanks!