In the Chrome tools, enabling this box:
This makes the debugger stop where an exception is thrown by the code (e.g when trying to call a function on an undefined object) so you can check values and the callstack at the point of the crash.
From here, I saw
this.freecells is virtually empty.
Now the next step is to work out where
this.freecells gets populated.
So somewhere from this code onwards,
this.freecells becomes empty.
Stepping through the code a few lines from here, you should be able to see where. I leave that as an exercise for you.