ERP5Site.py: in workflow migration, avoid calling accessor.

product/ERP5/ERP5Site.py

@@ -1673,7 +1673,7 @@ class ERP5Site(FolderMixIn, CMFSite, CacheCookieMixin):
         can only be executed after install bt erp5_workflow.
     """
     tool = self.portal_workflow
-    if tool.getPortalType() != 'Workflow Tool':
+    if not isinstance(tool, WorkflowTool()):
 
       # create new Workflow Tool
       new_tool = self._setObject('portal_workflow_new', WorkflowTool())