Fix wrong parent test in create_leveling

issue3246
review920002
(transplanted from 7705e9587201bdb38970df1c69d6ad3e325474a8)

work.py

@@ -596,9 +596,8 @@ class Work:
 
                 yield sibling, delay
 
-        parent = self.parent and self.parent.id or None
         siblings = self.search([
-                ('parent', '=', parent.id)
+                ('parent', '=', self.parent.id if self.parent else None)
                 ])
 
         refkey = get_key(self)
@@ -615,8 +614,8 @@ class Work:
                     'back_leveling_delay': delay,
                     })
 
-        if parent:
-            parent.compute_dates()
+        if self.parent:
+            self.parent.compute_dates()
 
     @classmethod
     def write(cls, works, values):