hierarchyItems property
A list of the types in the requested hierarchy. The first element of the list is the item representing the type for which the hierarchy was requested. The index of other elements of the list is unspecified, but correspond to the integers used to reference supertype and subtype items within the items.
This field will be absent if the code at the given file and offset does not represent a type, or if the file has not been sufficiently analyzed to allow a type hierarchy to be produced.
Implementation
List<TypeHierarchyItem>? hierarchyItems;