JavaScript runtime error: Unable to get property 'Value' of undefined or null reference

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C#

ASP.NET

Hello everyone,
I write a code for the dynamic text boxes to contain unique value but it is showing error as JavaScript runtime error: Unable to get property 'Value' of undefined or null reference
My code is as follows

for (j = 0; j <= 30; j++) {

             var id = "txt" + j

             var id1 = "txt" + (j + 1)
             var txtbox = document.getElementById(id);
             var txtbox1 = document.getElementById(id1);
             if (txtbox.Value == txtbox1.Value) {
                 alert("Enter unique serial number");
                 return false
             }