Fix server crash when lks_rootView is nil for macOS targets

[GetToSet]

Crash details:

UserAgent Queue : com.apple.libdispatch-io.opq (serial) EXC_BAD_ACCESS (code=1, address=0x0)
#0	0x000000019ef9f9a8 in swift_getObjectType ()
#1	0x0000000104572314 in static LKS_SwiftTraceManager.swiftMarkIVars(ofObject:) ()
#2	0x0000000104573f68 in @objc static LKS_SwiftTraceManager.swiftMarkIVars(ofObject:) ()
#3	0x00000001045455d0 in -[LKS_TraceManager _markIVarsInAllClassLevelsOfObject:] at /Users/ethanwong/Developer/LookInside/Sources/LookinServer/Server/Others/LKS_TraceManager.m:258
#4	0x00000001045442a8 in -[LKS_TraceManager _addTraceForLayersRootedByLayer:] at /Users/ethanwong/Developer/LookInside/Sources/LookinServer/Server/Others/LKS_TraceManager.m:108
#5	0x00000001045441ac in __26-[LKS_TraceManager reload]_block_invoke.18 at /Users/ethanwong/Developer/LookInside/Sources/LookinServer/Server/Others/LKS_TraceManager.m:83
#6	0x000000018b9bc548 in __NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__ ()
#7	0x000000018b9c005c in -[__NSFrozenArrayM enumerateObjectsWithOptions:usingBlock:] ()
#8	0x0000000104544000 in -[LKS_TraceManager reload] at /Users/ethanwong/Developer/LookInside/Sources/LookinServer/Server/Others/LKS_TraceManager.m:79
#9	0x000000010453fba8 in +[LKS_HierarchyDisplayItemsMaker itemsWithScreenshots:attrList:lowImageQuality:readCustomInfo:saveCustomSetter:] at /Users/ethanwong/Developer/LookInside/Sources/LookinServer/Server/Others/LKS_HierarchyDisplayItemsMaker.m:34
#10	0x00000001045464c8 in +[LookinHierarchyInfo(LookinServer) staticInfoWithLookinVersion:] at /Users/ethanwong/Developer/LookInside/Sources/LookinServer/Server/Others/LookinHierarchyInfo+LookinServer.m:31
#11	0x000000010452aac4 in -[LKS_RequestHandler handleRequestType:tag:object:] at /Users/ethanwong/Developer/LookInside/Sources/LookinServer/Server/Connection/LKS_RequestHandler.m:100
#12	0x0000000104529b10 in -[LKS_ConnectionManager ioFrameChannel:didReceiveFrameOfType:tag:payload:] at /Users/ethanwong/Developer/LookInside/Sources/LookinServer/Server/Connection/LKS_ConnectionManager.m:216
#13	0x000000010456c1c8 in __59-[Lookin_PTChannel startReadingFromConnectedChannel:error:]_block_invoke.46 at /Users/ethanwong/Developer/LookInside/Sources/LookinCore/Peertalk/Lookin_PTChannel.m:549
#14	0x000000010456e150 in __60-[Lookin_PTProtocol readPayloadOfSize:overChannel:callback:]_block_invoke at /Users/ethanwong/Developer/LookInside/Sources/LookinCore/Peertalk/Lookin_PTProtocol.m:300
#15	0x00000001016b4d58 in ___dispatch_operation_deliver_data_block_invoke ()
#16	0x00000001016804d4 in _dispatch_call_block_and_release ()
#17	0x000000010169cee0 in _dispatch_client_callout ()
#18	0x000000010168932c in _dispatch_lane_serial_drain ()
#19	0x000000010168a218 in _dispatch_lane_invoke ()
#20	0x00000001016c1358 in _dispatch_main_queue_drain.cold.6 ()
#21	0x00000001016919fc in _dispatch_main_queue_drain ()
#22	0x000000010169193c in _dispatch_main_queue_callback_4CF ()
#23	0x000000018ba0515c in __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ ()