[HELP]Failed to execute pipeline.

[David van Leerdam]

Description:org.apache.cocoon.ProcessingException: Failed to execute pipeline.: java.lang.RuntimeException: java.util.EmptyStackException
I have a stylesheet from which I have tried a dozen variations on the thema, but I can�t get it to work. Maybe any suggestions?

Thank you. David van Leerdam

Here is my code;

<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xsp="http://apache.org/xsp"
xmlns:ws="http://ws/logicsheet"
xmlns:esql="http://apache.org/cocoon/SQL/v2"
xmlns:xsp-request="http://apache.org/xsp/request/v2.0"
xmlns:xsp-session="http://apache.org/xsp/session/2.0"
create-session="true">

<xsp:logic>
public void getNewShoppingCart()
{
String s;

s.concat(session.getAttribute("shoppingcart"));
s.concat(request.getParameter("product_id");
s.concat(",");

session.setAttribute("shoppingcart", s);
}
</xsp:logic>

<xsl:template match="ws:product">

<esql:connection>
<esql:pool>portal</esql:pool>
<esql:execute-query>
<esql:query>
select * from producten where product_id = '<xsp:expr>request.getParameter("product_id")</xsp:expr>'
</esql:query>
<esql:results>
<esql:row-results>
<item>
<product_id><esql:get-string column="product_id"/></product_id>
<image><esql:get-string column="image"/></image>
<naam><esql:get-string column="naam"/></naam>
<prijs><esql:get-string column="prijs"/></prijs>
<voorraad><esql:get-string column="voorraad"/></voorraad>
</item>

<xsp:logic>
getNewShoppingCart();
</xsp:logic>
</esql:row-results>
</esql:results>
<esql:no-results>
<no-result>Het geselecteerde product kan niet worden gevonden.</no-result>
</esql:no-results>
</esql:execute-query>
</esql:connection>
</xsl:template>

<xsl:template match="@*|*|text()|processing-instruction()">
<xsl:copy>
<xsl:apply-templates select="@*|*|text()|processing-instruction()"/>
</xsl:copy>
</xsl:template>

</xsl:stylesheet>