As @paradigmatic states, it's not really a bug. It's the initialization order, which follows the declaration order. It this case, a is null when b is declared/init-ed.
Changing the line private val b = new B(a) to private lazy val b = new B(a) will fix issue, since using lazy will delay the init. of b to it's first usage.
It's very likely that this behavior is described in the SLS.
