cargo: don't crash.

OpenRa.Game/Traits/Cargo.cs

@@ -15,12 +15,14 @@ namespace OpenRa.Game.Traits
 
 		public Order IssueOrder(Actor self, int2 xy, MouseInput mi, Actor underCursor)
 		{
-			var unit = underCursor.traits.GetOrDefault<Unit>();
-			if (unit.Altitude > 0) return null;
-
 			// todo: check if there is an unoccupied `land` tile adjacent
 			if (mi.Button == MouseButton.Right && underCursor == self && cargo.Count > 0)
+			{
+				var unit = underCursor.traits.GetOrDefault<Unit>();
+				if (unit != null && unit.Altitude > 0) return null;
+
 				return new Order("Deploy", self, null, int2.Zero, null);
+			}
 
 			return null;
 		}