bug report: playing vs computer from set position isn't working

[TBestLittleHelper]

bug report: playing vs computer from set position isn't working while signed out. using firefox on windows 10. how to reproduce:

1. open https://lichess.org/?fen=r3k2r/pppqbppp/2npb3/8/5B2/2PB1N2/PPPQ1PPP/R3K2R_w_KQkq_-_7_10#ai while signed out (edit: or signed in, doesn't actually make a difference)
2. try changing difficulty level, or picking any colour to play with.
3. instead of a game starting, the play with the computer popup expands downwards with duplicates of the same board (edited) [19:31] I have a screenshot of the screen with several bugged boards in the popup but I don't think images can be posted here

in fact, this bug also occurs when accessing the link while signed in also. originally I wanted to just test the game behaviour when two click moves are disabled. I wanted to see if in a position where both sides castling is possible, if clicking on the king then clicking on a rook will result in castling (like what just happened to Anish Giri in his meltwater game vs Le Quang Liem).

Reported in Discord : https://discord.com/channels/280713822073913354/527867866700054538/968927177628344420 I can reproduce following the steps given.

[brollin]

Looks like chessground is breaking when it encounters a fen with underscores. I'll have a fix pretty quick.