The File object returned by FileSystemFileHandle.getFile() is only readable as long as the underlying file on disk hasn't changed. If the file on disk is modified, the File object becomes unreadable and you'll need to call getFile() again to get a new File object to read the changed data.
This seems very strange. Since when does a File stop being readable when it is modified? What does "modified" mean here anyways? Can anyone shed some light on this?