Assign Null value to the Integer Column in the DataTable

A null/empty string is in the wrong format; you would need to detect that scenario and compensate:

    DR["CustomerID"] = string.IsNullOrWhiteSpace(text)
        ? DBNull.Value : (object)Convert.ToInt32(text);

DR["CustomerID"] = !string.IsNullOrEmpty(TextBox1.Text)
                   ? Convert.ToInt32(TextBox1.Text)
                   : DBNull.Value;

But you should check also that the value is a valid integer:

int value;
if(int.TryParse(TextBox1.Text, out value))
{
    DR["CustomerID"] = value;
}
else
{
    DR["CustomerID"] = DBNull.Value;
}

you could do it like that:

DR["CustomerID"] = string.IsNullOrEmpty(TextBox1.Text) ?
    null : Convert.ToInt32(TextBox1.Text);

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