Fix crashes in WeaponInfo when warheads or projectiles cannot be created

OpenRA.Game/GameRules/WeaponInfo.cs

@@ -139,7 +139,11 @@ namespace OpenRA.GameRules
 		{
 			if (!yaml.ToDictionary().TryGetValue("Projectile", out var proj))
 				return null;
+
 			var ret = Game.CreateObject<IProjectileInfo>(proj.Value + "Info");
+			if (ret == null)
+				return null;
+
 			FieldLoader.Load(ret, proj);
 			return ret;
 		}
@@ -150,6 +154,9 @@ namespace OpenRA.GameRules
 			foreach (var node in yaml.Nodes.Where(n => n.Key.StartsWith("Warhead")))
 			{
 				var ret = Game.CreateObject<IWarhead>(node.Value.Value + "Warhead");
+				if (ret == null)
+					continue;
+
 				FieldLoader.Load(ret, node.Value);
 				retList.Add(ret);
 			}