Scala interpreter hangs if Spark is referenced before it is bound

Some users run cells in a notebook before the kernel is finished starting. The messages are queued and run after the KernelMessageRelay is enabled, but any messages that reference spark or bound variables cause IMain to throw an exception. That exception causes the interpreter to hang.

It seems odd that a missing variable reference causes IMain to throw an exception, so there may be a threading issue, a race between binding spark and the cell that references it.

The problem can be avoided by moving postInit tasks into init.