Object reference not set to an instance of an object. Request.Query String

Change

string ProductID = Request.QueryString["ID"].ToString(); 

to

 if(!string.IsNullOrWhiteSpace(Request.QueryString["ID"]))
 {
    string ProductID = Request.QueryString["ID"]; 
    DataList1.DataSource = showResult.ShowProductResult(ProductID);
 } 
 else
 {
    //Do something when Id is empty or null
    //DataList1.DataSource =null;
 }

Basically your getting that error because the page needs the missing information to complete its process.

If you want it to run without the error then you would need an "if" block to check if a (product) Id parameter has been passed in the querystring or url.

VB

If trim(request("ID")) <> "" Then
  'Do your product search here and display the results in your page HTML 
Else
  'No product ID parameter was passed! - You may optionally display this fact in your HTML
End If

C#

if (!string.IsNullOrEmpty(request("ID")) {
  //Do your product search here and display the results in your page HTML 
} else {
  //No product ID parameter was passed! - You may optionally display this fact in your HTML
}

Change

string ProductID = Request.QueryString["ID"].ToString(); 

to

 string ProductID="";
 if(Request.QueryString["ID"]!=null)
 {
    ProductID = Request.QueryString["ID"].ToString(); 
    DataList1.DataSource = showResult.ShowProductResult(ProductID);
 } 
 else
 {
    //DataList1.DataSource =null;
 }

Try this way,first check condition for query string return not null . see below code.

int ProductID ;
if(Request.QueryString["ID"]!=null)
 {
    ProductID =Convert.ToInt32( Request.QueryString["ID"].ToString()); 
    showResult.YourmethodName(ProductID);
 } 
 else
 {
    //Show one error lik "INVALID URL" ,Please go back
 }