Speculative fix for a crash if a handler cannot be found for a transaction.

Speculative fix for a crash if a handler cannot be found for a transaction.

Reported https://groups.google.com/d/msg/bluestatic/6NLJFmbEElg/hFtsRDX0CAAJ

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   org.bluestatic.macgdbp        	0x000000010cc0a316 -[ProtocolClient messageQueue:didReceiveMessage:] + 794
1   com.apple.CoreFoundation      	0x00007fff34763a6c __invoking___ + 140
2   com.apple.CoreFoundation      	0x00007fff34763911 -[NSInvocation invoke] + 303
3   com.apple.CoreFoundation      	0x00007fff34796eea -[NSInvocation invokeWithTarget:] + 70
4   org.bluestatic.macgdbp        	0x000000010cc0ab80 -[BSProtocolThreadInvoker dispatchInvocation:] + 123
5   com.apple.Foundation          	0x00007fff36e4f1bd __NSThreadPerformPerform + 204
6   com.apple.CoreFoundation      	0x00007fff34781f12 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
7   com.apple.CoreFoundation      	0x00007fff34781eb1 __CFRunLoopDoSource0 + 103
8   com.apple.CoreFoundation      	0x00007fff34781ccb __CFRunLoopDoSources0 + 209
9   com.apple.CoreFoundation      	0x00007fff347809fa __CFRunLoopRun + 927
10  com.apple.CoreFoundation      	0x00007fff3477fffe CFRunLoopRunSpecific + 462
11  com.apple.HIToolbox           	0x00007fff333b3abd RunCurrentEventLoopInMode + 292
12  com.apple.HIToolbox           	0x00007fff333b37d5 ReceiveNextEventCommon + 584
13  com.apple.HIToolbox           	0x00007fff333b3579 _BlockUntilNextEventMatchingListInModeWithFilter + 64
14  com.apple.AppKit              	0x00007fff319fec99 _DPSNextEvent + 883
15  com.apple.AppKit              	0x00007fff319fd4e0 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 1352
16  com.apple.AppKit              	0x00007fff319ef1ee -[NSApplication run] + 658
17  com.apple.AppKit              	0x00007fff319c0ff6 NSApplicationMain + 777
18  libdyld.dylib                 	0x00007fff6e6c8cc9 start + 1