Excuse me, I found that several objects' parentReceptacles is None after initialization.
The ai2thor's version is 4.0.0
Some reasons may be that the parentReceptacles belong to the room's structures. But other cases referring to SimOjects also return None parentReceptacles.
For example (actually, this happens in many scenes), in FloorPlan219, the Box|-02.31|+00.72|+00.35 outputs None parentReceptacles, while it is on the safe.
In FloorPlan309, both the CreditCard|-02.33|+01.10|-02.82 and Pen|-01.38|+01.11|-03.05 are on the Desk|-01.27|+01.17|-02.99, but the CreditCard's parentReceptacles is None, while the Pen's parentReceptacles returns the Desk.