Crash log when network connection becomes unavailable during package fetch

alcatraz_fetch_crash.log

Apr 24 20:21:01 M.local Xcode[79175]: [MT] DVTAssertions: UNCAUGHT EXCEPTION (NSInvalidArgumentException): data parameter is nil
  UserInfo: (null)
	Hints: None
Apr 24 20:21:01 M.local Xcode[79175]: [MT] DVTAssertions:
	Backtrace:
	  0  0x00007fff933fcaee __exceptionPreprocess (in CoreFoundation)
	  1  0x00007fff96c543f0 objc_exception_throw (in libobjc.A.dylib)
	  2  0x00007fff933fc8dc +[NSException raise:format:] (in CoreFoundation)
	  3  0x00007fff967eec61 +[NSJSONSerialization JSONObjectWithData:options:error:] (in Foundation)
	  4  0x0000000112c7615f __51-[ATZDownloader downloadPackageListWithCompletion:]_block_invoke at /Users/delisa/Code/OSS/alcatraz/Alcatraz/Helpers/ATZDownloader.m:34 (in Alcatraz)
	  5  0x00007fff966c512f -[NSBlockOperation main] (in Foundation)
	  6  0x00007fff9669b036 -[__NSOperationInternal start] (in Foundation)
	  7  0x00007fff966a2861 __block_global_6 (in Foundation)
	  8  0x00007fff8d454f01 _dispatch_call_block_and_release (in libdispatch.dylib)
	  9  0x00007fff8d4510b6 _dispatch_client_callout (in libdispatch.dylib)
	 10  0x00007fff8d4560c8 _dispatch_main_queue_callback_4CF (in libdispatch.dylib)
	 11  0x00007fff9339eb4c __CFRunLoopRun (in CoreFoundation)
	 12  0x00007fff9339e0e2 CFRunLoopRunSpecific (in CoreFoundation)
	 13  0x00007fff8f17eeb4 RunCurrentEventLoopInMode (in HIToolbox)
	 14  0x00007fff8f17ec52 ReceiveNextEventCommon (in HIToolbox)
	 15  0x00007fff8f17eae3 BlockUntilNextEventMatchingListInMode (in HIToolbox)
	 16  0x00007fff8c5f9563 _DPSNextEvent (in AppKit)
	 17  0x00007fff8c5f8e22 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] (in AppKit)
	 18  0x00007fff8c5f01d3 -[NSApplication run] (in AppKit)
	 19  0x00007fff8c594c06 NSApplicationMain (in AppKit)
	 20  0x000000010bf54b6f (in Xcode)
	 21  0x000000010bf54b00 (in Xcode)