You can’t add a constraint to a single body / aka to itself.
I’m guessing the out-of-memory error is a recursive loop of sorts.
As discovered here:
You can use a constraint as you are doing now, but with a parent
Or you can set the character “imposter’s” velocity to 0
@Cedric mentioned in the topic above that Havoc comes with a built-in character controller aswell!
But it won’t be in BabylonJs yet for some months.