guarded against UniqueInteger undefined usage

Author: bahusoid

src/NHibernate/Mapping/Table.cs

@@ -30,7 +30,7 @@ public class Table : IRelationalModel
 		private readonly LinkedHashMap<string, Column> columns = new LinkedHashMap<string, Column>();
 		private readonly Dictionary<ForeignKeyKey, ForeignKey> foreignKeys = new Dictionary<ForeignKeyKey, ForeignKey>();
 		private readonly Dictionary<string, Index> indexes = new Dictionary<string, Index>();
-		private int uniqueInteger;
+		private int? uniqueInteger;
 		private readonly Dictionary<string, UniqueKey> uniqueKeys = new Dictionary<string, UniqueKey>();
 		private string catalog;
 		private string comment;
@@ -189,7 +189,7 @@ public string Schema
 		/// <value>The unique number of the Table.</value>
 		public int UniqueInteger
 		{
-			get { return uniqueInteger; }
+			get { return uniqueInteger ?? throw new InvalidOperationException(nameof(UniqueInteger) + " has not been supplied"); }
 			internal set { uniqueInteger = value; }
 		}