get the higher item in a product structure based on several ItemTypes

Hi Community, I don't understand what methods like getRelationships or getParentItem() do as I always get the same error when I use it: "Object reference not set to an instance of an object." What type of data is returned by these methods? I am looking for a way to go up in a product structure built using several ItemTypes (not like the standard solution where all is based on the "Part" ItemType with its class structure, in my case classes are made with ItemTypes). the structure is made like this: Part---partInstance--->Assy---AssyInstance--->Assy---AssyInstance--->Root How to go up until Root from a server event running on partInstance? Best regards, Camille