Use preconditions

Author: sovdeeth

src/main/java/ch/njol/skript/classes/Changer.java

@@ -1,5 +1,6 @@
 package ch.njol.skript.classes;
 
+import com.google.common.base.Preconditions;
 import org.bukkit.event.Event;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
@@ -104,17 +105,18 @@ public static boolean acceptsChangeTypes(Class<?>[] validTypes, Class<?> @NotNul
 		 * Gets the types that can be added/removed via arithmetic for the given type.
 		 * This is used to determine accepted change types for add/remove when no changer is present.
 		 * @param type The type to get arithmetic change types for.
-		 * @param mode Whether to get addition or subtraction types.
+		 * @param mode Whether to get addition or subtraction types. Only {@link ChangeMode#ADD} and {@link ChangeMode#REMOVE} are supported.
 		 * @param filter A filter to apply to the available operations. Used for custom constraints on the operations, like
 		 *               ensuring the return type matches the left type.
 		 * @return The types that can be added/removed via arithmetic for the given type and mode, after applying the filter.
 		 * @param <T> The type to get arithmetic change types for.
 		 */
 		public static <T> Class<?>[] getArithmeticChangeTypes(Class<T> type, ChangeMode mode, Predicate<OperationInfo<T, ? ,?>> filter) {
-			List<OperationInfo<T, ?, ?>> opInfos = List.of();
+			Preconditions.checkState(mode == ChangeMode.ADD || mode == ChangeMode.REMOVE, "Only ADD and REMOVE modes are supported for arithmetic change types");
+			List<OperationInfo<T, ?, ?>> opInfos;
 			if (mode == ChangeMode.ADD) {
 				opInfos = Arithmetics.getOperations(Operator.ADDITION, type);
-			} else if (mode == ChangeMode.REMOVE) {
+			} else {
 				opInfos = Arithmetics.getOperations(Operator.SUBTRACTION, type);
 			}
 			return opInfos.stream()