Fixed failing SiteServiceTest.

git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/HEAD/root@30025 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261

source/java/org/alfresco/repo/workflow/jbpm/JBPMEngine.java

@@ -2240,6 +2240,10 @@ public class JBPMEngine extends AlfrescoBpmEngine implements WorkflowEngine
                     // retrieve process instance
                     GraphSession graphSession = context.getGraphSession();
                     ProcessInstance processInstance = getProcessInstanceIfExists(graphSession, workflowInstanceId);
+                    if(processInstance == null)
+                    {
+                        return null;
+                    }
                     Task startTask = processInstance.getProcessDefinition().getTaskMgmtDefinition().getStartTask();
                     
                     // retrieve task