WorklistVariable.py: deploy property accessors as many as possible.

product/ERP5Workflow/Document/WorklistVariable.py

@@ -66,7 +66,8 @@ class WorklistVariable(IdAsReferenceMixin("variable_", "prefix"), XMLObject):
     )
 
     def getDefaultExprText(self):
-        if not self.default_expr:
+        default_expr = self.getDefaultExpr()
+        if not default_expr:
             return ''
         else:
-            return self.default_expr.text
+            return default_expr