Lighting Build Fails With High Density Foliage

Trying to build lighting on a map with high density foliage will fail lighting build or cause the editor to crash. This occurs in 4.15, 4.16, 4.17, and 4.18. There have been multiple reports of this bug on AnswerHub. 

One user reports that after the lighting build fails in 4.18, they open a new project in 4.14, build lighting, then switch back to the scene from 4.18 (with Swarm agent from 4.14 still open) and lighting will build successfully. 

2. Create new default empty scene
3. Scale floor to have room to paint foliage - 15x15x1
4. Use simple mesh like /Engine/BasicShapes/Cube as the static mesh for Foliage 
5. Change Painting > Density / 1Kuu to 5000
6. Cover floor with painted foliage
7. Build Lighting

Result: Lighting will fail or engine will crash.

12:13:41 PM: Starting up SwarmAgent ...
12:13:41 PM: ... registering SwarmAgent with remoting service
12:13:41 PM: ... registering SwarmAgent network channels
12:13:41 PM: ... initializing SwarmAgent
12:13:41 PM: ......... certificate check has failed
12:13:41 PM: ...... initializing cache
12:13:41 PM: ......... using cache folder 
12:13:41 PM: ......... recreating SwarmAgent cache staging area
12:13:41 PM: ...... initializing connection to SwarmCoordinator
12:13:41 PM: ......... using SwarmCoordinator on RENDER-01
12:13:41 PM: ......... SwarmCoordinator successfully initialized
12:13:41 PM: ...... initializing local performance monitoring subsystem
12:13:41 PM: ... initialization successful, SwarmAgent now running
12:13:41 PM: [Interface:TryOpenConnection] Local connection established
12:16:54 PM: [Job] Accepted Job 4D44E331-4D2F001D-BB7EB79F-C320942B
12:28:07 PM: [Interface:EndJobSpecification] Error: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
Server stack trace:
at System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument, ExceptionResource resource)
at Agent.AgentJob.EndJobSpecification()
at Agent.Agent.EndJobSpecification_1_0(Int32 ConnectionHandle, Hashtable InParameters, Hashtable& OutParameters)
at System.Runtime.Remoting.Messaging.StackBuilderSink._PrivateProcessMessage(IntPtr md, Object[] args, Object server, Object[]& outArgs)
at System.Runtime.Remoting.Messaging.StackBuilderSink.SyncProcessMessage(IMessage msg)
Exception rethrown at [0]:
at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
at NSwarm.IAgentInterfaceWrapper.EndJobSpecificationDelegate.EndInvoke(Hashtable& OutParameters, IAsyncResult result)
at NSwarm.IAgentInterfaceWrapper.EndJobSpecification(Int32 ConnectionHandle)
at NSwarm.FSwarmInterface.EndJobSpecification()