Return value using String result=Command.ExecuteScalar() error occurs when result returns null

Question

There is no need to keep calling .ToString() as getValue is already a string.

Aside that, this line could possibly be your problem:

 string getValue = cmd.ExecuteScalar().ToString();

If there are no rows .ExecuteScalar will return null so you need to do some checking.

For instance:

var firstColumn = cmd.ExecuteScalar();

if (firstColumn != null) {
    result = firstColumn.ToString();
}