See :http://www.jboss.org/index.html?module=bb&op=viewtopic&t=159414 for a description of the issue.

Here's a process description snippet:

<?xml version="1.0" encoding="UTF-8"?>

<process name="GTRIHelpRequest" xmlns="http://jbpm.org/4.0/jpdl">

<on event="start">
<event-listener class="gtri.jbpm.pilot.LogListener">
<field name="msg"><string value="starting the process GTRIHelpRequest"/></field>
</event-listener>
</on>

<start form="gtri/jbpm/pilot/demo/request_help.ftl" g="0,0,80,40" name="start">
<on event="start">
<event-listener class="gtri.jbpm.pilot.LogListener">
<field name="msg"><string value="In the start activity - start event - of GTRIHelpRequest process"/></field>
</event-listener>
</on>
<transition to="debug_variables"/>
</start>

<script expr="Just dumping process variables" g="66,10,80,40" name="debug_variables" var="processVarABC">
<on event="start">
<event-listener class="gtri.jbpm.pilot.LogListener">
<field name="msg"><string value="In the debug variables activity - start event - of GTRIHelpRequest process"/></field>
</event-listener>
</on>
<transition to="is_lab_approval_needed"/>
</script>

<decision g="354,85,80,40" name="is_lab_approval_needed">
<on event="start">
<event-listener class="gtri.jbpm.pilot.LogListener">
<field name="msg"><string value="In the is_lab_approval activity - start event - of GTRIHelpRequest process"/></field>
</event-listener>
</on>
<transition to="in_lab_approval_email">
<condition expr="#

{(hr_subcategory=='newhire') || (hr_subcategory=='newconst') || (hr_subcategory=='badge') }"/>
</transition>
<transition to="in_request_handling_email"/>
</decision>

We have a form tied to the start activity. In this form(request_help.ftl) , we submit a param hr_subcategory:
<tr>
<td>Sub-categories:</td>
<td>
<select name="hr_subcategory">
<option value="newhire">New Hire</option>
<option value="others">Others</option>
</select>
</td>
<tr>

We transition to the is_lab_approval_needed decision after a debug activity. In the decision, is_lab_approval_needed, we try to reference the hr_subcategory value as follows in one of the conditions:
<condition expr="#{(hr_subcategory=='newhire') || (hr_subcategory=='newconst') || (hr_subcategory=='badge') }

"/>

This reference causes a Null pointer to be thrown back on the BPM workflow console.

Now introduce a halt in the workflow before the decision eg :

<script expr="Just dumping process variables" g="66,10,80,40" name="debug_variables" var="processVarABC">
<on event="start">
<event-listener class="gtri.jbpm.pilot.LogListener">
<field name="msg"><string value="In the debug variables activity - start event - of GTRIHelpRequest process"/></field>
</event-listener>
</on>
<transition to="manager_approval"/>
</script>

<task candidate-users="peter,mary" form="gtri/jbpm/pilot/demo/manager_approval.ftl" g="163,10,131,40" name="manager_approval">
<on event="start">
<event-listener class="gtri.jbpm.pilot.LogListener">
<field name="msg"><string value="In the manager_approval_test activity - start event - of GTRIHelpRequest process"/></field>
</event-listener>
</on>
<notification/>
<transition g="-38,-18" name="Allowed" to="is_lab_approval_needed"/>
<transition g="-38,-18" name="Sorry" to="request_rejected"/>
</task>

<decision g="354,85,80,40" name="is_lab_approval_needed">
<on event="start">
<event-listener class="gtri.jbpm.pilot.LogListener">
<field name="msg"><string value="In the is_lab_approval activity - start event - of GTRIHelpRequest process"/></field>
</event-listener>
</on>
<transition to="in_lab_approval_email">
<condition expr="#

{(hr_subcategory=='newhire') || (hr_subcategory=='newconst') || (hr_subcategory=='badge') }

"/>
</transition>
<transition to="in_request_handling_email"/>
</decision>

The NPE is resolved - the hr_subcategory is now visible in the decision after the manager approval.

The params submitted via the taskform linked with the start activity should be available in any subsequent activity for reference in a expression ..without having to impose a halt in the workflow like we had to.