Repeated "Uncaught TypeError: Cannot set properties of undefined (setting '0')" prevents testing the game

[jontopielski]

Hi, I seem to get the following error every ~10 minutes or so of using the engine and have to refresh to be able to hit play:

If I try to play, the window just gets stuck with this:

Also, I notice every animation jitters extremely quickly. Not sure if that might be useful for debugging. Thanks for taking a look!