Handle TextLines.Insert when index equals number of items

Author: Kapil Borle

Engine/TextLines.cs

@@ -86,12 +86,7 @@ public string this[int index]
         /// <param name="item">A non null object of type String.</param>
         public void Add(string item)
         {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            Insert(Count - 1, item);
+            Insert(Count, item);
         }
 
         /// <summary>
@@ -165,6 +160,10 @@ public void Insert(int index, string item)
             {
                 itemInserted = lines.AddFirst(item);
             }
+            else if (Count == index)
+            {
+                itemInserted = lines.AddLast(item);
+            }
             else
             {
                 ValidateIndex(index);