Error in Filter functionality on Dispaly method field on the form

Hi all,

      I write following code in the context method on form control for the filter functionality. But when I click remove filter then following error occurs.

"The required QueryBuildDataSource was not found in the Query associated with the FormDataSource '%1 "

and code is -

int             selectedMenu;
    formrun         fr;
    Args            ag;
    Name            strtext;
    querybuilddataSource qb1;
    queryrun    qr;
    query       q;
    PopupMenu menu = new PopupMenu(element.hWnd());
    int a = menu.insertItem('Filter By Field');
    int b = menu.insertItem('Filter By Selection');
    int c = menu.insertItem('Remove Filter');
    ;

    selectedMenu = menu.draw();
    switch (selectedMenu)
    {
    case -1: //Filter by field
            break;
    case a:
            ag = new args('SysformSearch');
            fr = new formrun(ag);
            fr.run();
            fr.wait();
//Reading User entered value for filter process
            strtext = fr.design().controlName('FindEdit').valueStr();
            if(strtext)
            {
//Creating a query for filter
                q   = PurchaseTable_ds.query();
                //q   = smmBusRelTable_ds.query();
                qb1 = q.dataSourceTable(tablenum(PurchTable));
                //qb1 = q.dataSourceTable(tablenum(smmBusRelTable));
                qb1 = qb1.addDataSource(TableNum(VendPackingSlipJour));
                //qb1 = qb1.addDataSource(TableNum(CustTable));
                qb1.addLink(FieldNum(PurchTable,PurchId),FieldNum(VendPackingSlipJour,PurchId));
                //qb1.addLink(FieldNum(smmBusRelTable,CustAccount),FieldNum(CustTable,AccountNum));
                qb1.addRange(FieldNum(VendPackingSlipJour,GRNNumber_IN)).value(strtext);
                //qb1.addRange(FieldNum(CustTable,Name)).value(strtext);
                PurchaseTable_ds.query(Q);
                //smmBusRelTable_ds.query(Q);
                PurchaseTable_ds.executeQuery();
                //smmBusRelTable_ds.executeQuery();
            }
            break;



    case b:   // Filter By Selection
            q   = PurchaseTable_ds.query();
            //q   = smmBusRelTable_ds.query();
            qb1 = q.dataSourceTable(tablenum(PurchTable));
            //qb1 = q.dataSourceTable(tablenum(smmBusRelTable));
            qb1 = qb1.addDataSource(TableNum(VendPackingSlipJour));
            qb1.addLink(FieldNum(PurchTable,PurchId),FieldNum(VendPackingSlipJour,PurchId));
            qb1.addRange(FieldNum(VendPackingSlipJour,PurchId)).value(GRN_Number.valueStr());

            PurchaseTable_ds.query(Q);
            PurchaseTable_ds.executeQuery();
            break;

    case c :   // Remove Filter
            q   = new Query();
            qb1 = q.addDataSource(tablenum(PurchTable));
            qb1.clearLinks();
            qb1.clearRanges();
            PurchaseTable_ds.query(Q);
            PurchaseTable_ds.removeFilter();

            break;

    Default:
            break;
    }