Fix Nullable<T> configs

NeosModLoader/ModConfiguration.cs

@@ -363,8 +363,7 @@ public void Set(ModConfigurationKey key, object? value, string? eventLabel = nul
 
 			if (value == null)
 			{
-				bool cannotBeNull = definingKey!.ValueType().IsValueType && Nullable.GetUnderlyingType(definingKey!.ValueType()) == null;
-				if (cannotBeNull)
+				if (Util.CannotBeNull(definingKey!.ValueType()))
 				{
 					throw new ArgumentException($"null cannot be assigned to {definingKey.ValueType()}");
 				}

NeosModLoader/ModConfigurationKey.cs

@@ -136,13 +136,27 @@ public ModConfigurationKey(string name, string? description = null, Func<T>? com
 		/// <returns>true if the value is valid</returns>
 		public override bool Validate(object? value)
 		{
-			// specifically allow nulls for class types
-			if (value is T || (value is null && !typeof(T).IsValueType))
+			if (value is T typedValue)
 			{
-				return ValidateTyped((T?)value);
+				// value is of the correct type
+				return ValidateTyped(typedValue);
+			}
+			else if (value == null)
+			{
+				if (Util.CanBeNull(ValueType()))
+				{
+					// null is valid for T
+					return ValidateTyped((T?)value);
+				}
+				else
+				{
+					// null is not valid for T
+					return false;
+				}
 			}
 			else
 			{
+				// value is of the wrong type
 				return false;
 			}
 		}

NeosModLoader/Util.cs

@@ -90,5 +90,17 @@ internal static HashSet<T> ToHashSet<T>(this IEnumerable<T> source, IEqualityCom
 			return new HashSet<T>(source, comparer);
 		}
 
+		// check if a type cannot possibly have null assigned
+		internal static bool CannotBeNull(Type t)
+		{
+			return t.IsValueType && Nullable.GetUnderlyingType(t) == null;
+		}
+
+		// check if a type is allowed to have null assigned
+		internal static bool CanBeNull(Type t)
+		{
+			return !CannotBeNull(t);
+		}
+
 	}
 }