Uncaught error when opening multiple documents in MacDown on Mac OS X 10.11 beta

1. Run MacDown: the default blank document opens.
2. Choose File > Open to open another document.

EXPECTED: The new document opens, and the default blank document is still there. (Incidentally I think it would be a good enhancement to close the default blank document when opening another document, if the default blank document has no unsaved changes.)

ACTUAL: The opened document window appears, but a message box appears reporting a caught exception, and saying that you can click Continue to run in an inconsistent state, but woe betide you.

1. Click Continue.

EXPECTED: Not sure. Hopefully I can keep working! ACTUAL: MacDown crashes out.

WORKAROUND: Close the default document before opening the document you want to edit, and only have one document window open at once.