Add error type and null type.

Assets/Cryville/Common/Pdt/PdtInternalType.cs

@@ -3,12 +3,19 @@
 	/// The identifiers of the internal types of PDT.
 	/// </summary>
 	public static class PdtInternalType {
-		internal readonly static int Error = 0x00525245;
+		/// <summary>
+		/// Error type.
+		/// </summary>
+		public readonly static int Error = 0x00525245;
 		/// <summary>
 		/// Array of a same variable-length type, with a suffix indicating the element count and the element type.
 		/// </summary>
 		public readonly static int Array = 0x00525241;
 		/// <summary>
+		/// Null type.
+		/// </summary>
+		public readonly static int Null = 0x4c4c554e;
+		/// <summary>
 		/// IEEE 754 32-bit floating-point number.
 		/// </summary>
 		public readonly static int Number = 0x004d554e;