#8 Null Pointer exception if parent ack arrives too fast

[Goutham Bharadwaj]

NullPointer exception is thrown on spread.getDfmHandler
() if the ack for the first parent arrives before the
spread.startSpread() call has completed. (This creates
the DFMHandlers)

As Simulator and Spread run in different threads this
possible when ack for first parent (Generated by
SimulatorMarketThread) arrives before DFMHandlers are
completely created (MainThread)