fixed

Sieve/Models/RemoveVietNamSign.cs

@@ -41,6 +41,10 @@ namespace Sieve.Models
            };
         public string RemoveSignVietnameseString(string str)
         {
+            if (str==null)
+            {
+                return "";
+            }
             for (int i = 1; i < VietnameseSigns.Length; i++)
             {
                 for (int j = 0; j < VietnameseSigns[i].Length; j++)