button 'create equipment' invisible when purchase not has invoice

purchase.py

@@ -21,7 +21,9 @@ class Purchase(metaclass=PoolMeta):
         super(Purchase, cls).__setup__()
         cls._buttons.update({
             'create_equipments': {
-                'invisible': If(Bool(Eval('equipment_create')), True)},
+                'invisible':  If(Eval('invoice_state') == 'none', True) |
+                              If(Bool(Eval('equipment_create')), True),
+                'depends': ['invoice_state'],}   
         })
 
     @classmethod