The windows of the petrol-bombed police car were attacked while an officer was still inside the vehicle.
A police officer was also waiting outside the vehicle. Both officers escaped unhurt.
More top news
The Friends star had reportedly not seen her mother for several years following a feud - but reconnected with her shortly before she died.
A deal is believed to have been reached with official confirmation likely on Friday.
The woman is described as being around 50 years old with short spikey purple hair.