fix crash on telling {sam,agun} to attack ground

OpenRA.Mods.RA/AttackBase.cs

@@ -257,6 +257,6 @@ namespace OpenRA.Mods.RA
 		public bool HasAnyValidWeapons(Target t) { return Weapons.Any(w => w.IsValidAgainst(t)); }
 		public float GetMaximumRange() { return Weapons.Max(w => w.Info.Range); }
 
-		public Weapon ChooseWeaponForTarget(Target t) { return Weapons.First(w => w.IsValidAgainst(t)); }
+		public Weapon ChooseWeaponForTarget(Target t) { return Weapons.FirstOrDefault(w => w.IsValidAgainst(t)); }
 	}
 }

OpenRA.Mods.RA/AttackTurreted.cs

@@ -51,6 +51,8 @@ namespace OpenRA.Mods.RA
 			const int RangeTolerance = 1;	/* how far inside our maximum range we should try to sit */
 			/* todo: choose the appropriate weapon, when only one works against this target */
 			var weapon = ChooseWeaponForTarget(Target.FromOrder(order));
+			if (weapon == null)
+				return;
 
 			target = Target.FromOrder(order);