Transition.py: fix guard generator.

product/ERP5Workflow/Document/Transition.py

@@ -93,16 +93,17 @@ class Transition(IdAsReferenceMixin("transition_", "prefix"), XMLObject):
 
   def generateGuard(self):
     if self.trigger_type == TRIGGER_USER_ACTION:
-      if self.guard == None:
+      if self.guard is None:
         self.guard = Guard(permissions=self.getPermissionList(),
                       roles=self.getRoleList(),
                       groups=self.getGroupList(),
-                      expr=self.getExpression())
-      if self.guard.roles != self.getRoleList():
+                      expr=Expression(self.getExpression()))
+
+      if self.getRoleList() is not None:
         self.guard.roles = self.getRoleList()
-      if self.guard.permissions != self.getPermissionList():
+      if self.getPermissionList() is not None:
         self.guard.permissions = self.getPermissionList()
-      if self.guard.groups != self.getGroupList():
+      if self.getGroupList() is not None:
         self.guard.groups = self.getGroupList()
-      if self.guard.expr != self.getExpression():
-        self.guard.expr = self.getExpression()
+      if self.getExpression() is not None:
+        self.guard.expr = Expression(self.getExpression())