If it is happening on this line
string a = Request.Params.ToString();
then either
Request
has not been initialised or does not contain
Params
So your problem is not in this code, but elsewhere in your form, before you get to the point where you click the button.
Also
Params
returns a collection so
Request.Params.ToString();
will just return descriptive text, not any value.
Further reference with an example...
at msdn[
^]