Crash with special characters

[Pitasi]

Hi there, I just wanted to report a bug that happens when some special characters are used. In my case, I think the bad characters are some emojis I used in my book.

From the javascript console:
Cannot read property 'tf' of undefined

Adding this to book.json fixed the problem.

{
    "pluginsConfig": {
        "lunr": {
            "ignoreSpecialCharacters": true
        }
    }
}