SqlDataSource SelectCommand using LIKE does not work

I have the following T-SQL in a SelectCommand:

SELECT h.Business,
hrl.frn
FROM registration hrl
INNER JOIN holder h on h.call = hrl.call
WHERE 
(h.Business like '%' + @business + '%' and h.Business is not null) 
and 
(hrl.frn = @frn and hrl.frn is not null)

business and frn are tied to control parameters and it should return data even if one or both is left blank, but if I put in data just for frn for example, it does not return anything. I think my T-SQL is not doing the right thing and I am also not sure if I am handling the like correctly.

if both textboxes are left empty, it should return all data. If frn is entered, but business is left blank, it should only return data related to that frn. If business if entered, but frn is left blank, it should return all matches like business. If both are entered, it should return data only matching the frn and the business.

Also, I am not sure if doing the and is not null is actually necessary.

protected void btnSearch_Click(object sender, EventArgs e)
{
    if (txtFRN.Text == "")
        frn = null;

    if (txtBusiness.Text == "")
        business = null;

    sqlDsMaster.SelectParameters[frn].DefaultValue = frn;
    sqlDsMaster.SelectParameters[business].DefaultValue = business;

    sqlDsMaster.DataBind();
}

The above throws an "Object Reference not set to an instance" when it hits this line:

sqlDsMaster.SelectParameters[frn].DefaultValue = frn;

frn and business are properties.

Here is the SearchMaster stored procedure:

CREAETE PROCEDURE SearchMaster
@business nvarchar(300) = NULL,
@frn nvarchar(10) = NULL
AS
SELECT h.Business,
       hrl.frn
FROM registration hrl
INNER JOIN holder h on h.call = hrl.call
WHERE (@business IS NULL OR h.Business like '%' + @business + '%') 
  AND (@frn IS NULL OR hrl.frn = @frn)

Here is the SearchDetails stored procedure:

CREATE PROCEDURE SearchDetails
@business nvarchar(300) = NULL,
@frn nvarchar(10) = NULL
AS
SELECT hrl.call 
FROM registration hrl 
INNER JOIN holder h ON h.call = hrl.call
WHERE (@business IS NULL OR h.Business LIKE '%' + @business + '%') 
      AND (@frn IS NULL OR hrl.frn = @frn)

Here is the SqlDataSource for the SearchMaster procedure:

<asp:SqlDataSource ID="sqlDsDetails" 
                   runat="server" 
                   ConnectionString="<%$ ConnectionStrings:cnxString %>
                   SelectCommandType="StoredProcedure" 
                   SelectCommand="SearchMaster">
  <SelectParameters>
    <asp:ControlParameter Name="business" ControlID="txtBusiness" 
                          Type="String" PropertyName="Text"  
                          ConvertEmptyStringToNull="true" />
    <asp:ControlParameter Name="frn" ControlID="txtFRN" 
                          Type="String" PropertyName="Text"
                          ConvertEmptyStringToNull="true"/>
  </SelectParameters>
</asp:SqlDataSource>

Here is the SqlDataSource for the SearchDetails procedure:

<asp:SqlDataSource ID="sqlDsDetails" 
                   runat="server" 
                   ConnectionString="<%$ ConnectionStrings:cnxString %>
                   SelectCommandType="StoredProcedure" 
                   SelectCommand="SearchDetails">
  <SelectParameters>
    <asp:Parameter Name="frn" Type="String" DefaultValue="" 
                   ConvertEmptyStringToNull="true" />
    <asp:Parameter Name="business" Type="String" DefaultValue="" 
                   ConvertEmptyStringToNull="true" />
  </SelectParameters>
</asp:SqlDataSource>

Here is the button click that binds the SqlDsMaster:

protected void btnSearch_Click(object sender, EventArgs e)
{
    sqlDsMaster.DataBind();
}

Here is the gvMaster_RowCreated that creates the rows for the details:

protected void gvMaster_RowCreated(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        SqlDataSource ctrl = 
        e.Row.FindControl("sqlDsDetails") as SqlDataSource;

        if (ctrl != null && e.Row.DataItem != null)
        {
            ctrl.SelectParameters["frn"].DefaultValue = 
            ((DataRowView)e.Row.DataItem)["frn"].ToString();

            ctrl.SelectParameters["business"].DefaultValue = 
            ((DataRowView)e.Row.DataItem)["business"].ToString();
         }
     }
 }

SearchMaster and SearchDetails both work if I run it through SQL Server Management Studio and it works if I enter both data for business and frn, but if I enter just one, no data is returned. Are the parameters set up correctly? Also, if I am initializing the parameters to null in the procedure, is it still necessary to use ConvertEmptyStringToNull?