start hacking up lobby to work with slots (cnc only)

OpenRA.Game/Traits/Player/TechTreeCache.cs

@@ -34,6 +34,9 @@ namespace OpenRA.Traits
 
 			public void Tick( Player owner, Cache<string, List<Actor>> buildings )
 			{
+				if (owner.Country == null)
+					return;
+
 				var effectivePrereq = prerequisites.Where( a => a.Traits.Get<BuildableInfo>().Owner.Contains( owner.Country.Race ) );
 				var nowHasPrerequisites = effectivePrereq.Any() &&
 					effectivePrereq.All( a => buildings[ a.Name ].Any( b => !b.Trait<Building>().Disabled ) );