Spring WebFlow, empty value from requestParameters

Hari ni berhadapan dengan isu parameter yang dibaca daripada view tidak ada value.
Error yang terlibat adalah NullPointerException kalau tak silap.

Framework

SPRING WebFlow + JSP

Kod asal:

      <view-state id="view_superviseeStudyPlanListing" view="studyplanfrm/summaryForSVor" model="studyPlanForm">  
           <on-entry>  
                <!-- should have a decision-state to check eligibility -->  
                <!-- load list of supervisee -->  
                <evaluate expression="studyPlanFrmService.loadSummaryForSupervisor(flowScope.studyPlanForm)" result="flowScope.studyPlanForm"></evaluate>  
           </on-entry>  
           <transition on="approval" to="view_studyPlanMainForSupervisor" >  
                <evaluate expression="requestParameters.ssp" result="requestScope.paramStaffStudyPlanCode" />       
           </transition>  
      </view-state>  
      <view-state id="view_studyPlanMainForSupervisor" view="studyplanfrm/studyPlanFrmMain" model="studyPlanForm">  
      <!-- sharing view with view_studyPlanMain -->  
           <on-entry>  
                <!-- load main form with all related contents -->  
                <evaluate expression="studyPlanFrmService.loadMainFormForSupervisor( studyPlanForm, paramStaffStudyPlanCode )" result="flowScope.studyPlanForm" >  
                </evaluate>  
                <set name="flowScope.msj" value="flowScope.studyPlanForm.formMessage" ></set>  
           </on-entry>  
      </view-state>  

Kod terbaru:

      <view-state id="view_superviseeStudyPlanListing" view="studyplanfrm/summaryForSVor" model="studyPlanForm">  
           <on-entry>  
                <!-- should have a decision-state to check eligibility -->  
                <!-- load list of supervisee -->  
                <evaluate expression="studyPlanFrmService.loadSummaryForSupervisor(flowScope.studyPlanForm)" result="flowScope.studyPlanForm"></evaluate>  
           </on-entry>  
           <transition on="approval" to="view_studyPlanMainForSupervisor" >  
           </transition>  
      </view-state>  
      <view-state id="view_studyPlanMainForSupervisor" view="studyplanfrm/studyPlanFrmMain" model="studyPlanForm">  
      <!-- sharing view with view_studyPlanMain -->  
           <on-entry>  
                <evaluate expression="requestParameters.ssp" result="requestScope.paramStaffStudyPlanCode" />       
                <!-- load main form with all related contents -->  
                <evaluate expression="studyPlanFrmService.loadMainFormForSupervisor( studyPlanForm, paramStaffStudyPlanCode )" result="flowScope.studyPlanForm" >  
                </evaluate>  
                <set name="flowScope.msj" value="flowScope.studyPlanForm.formMessage" ></set>  
           </on-entry>  
      </view-state>  

Penyelesaian?

Ermm, selepas pindahkan evaluation untuk requestParameters yang berkenaan semua nampak ok.