...

Back/Data/Models/Invoice.cs

@@ -148,7 +148,7 @@ namespace Back.Data.Models
         {
             get
             {
-                return pattern.BillTypeID == 3 || (pattern.BillTypeID == 2 && pattern.ID == 10)
+                return pattern!=null && (pattern.BillTypeID == 3 || (pattern.BillTypeID == 2 && pattern.ID == 10))
                     ? 1
                     : _setm;
             }

Back/Data/Models/InvoiceItem.cs

@@ -4,7 +4,7 @@ using System.Reflection.Metadata.Ecma335;
 
 namespace  Back.Data.Models
 {
-    public class InvoiceItem
+    public class InvoiceItem : ICloneable
     {
         #region Key
         public int ID { get; set; }
@@ -147,6 +147,11 @@ namespace  Back.Data.Models
 
         [ForeignKey("CODID")]
         public virtual CODItem cODItem { get; set; }
+
+        public object Clone()
+        {
+            return this.MemberwiseClone();
+        }
         #endregion
 
     }