rapidjson

avoid reference to null pointer and member access within null pointer

UBSAN gave issues with the typeless Schema:
runtime error: reference binding to null pointer of type 'rapidjson::GenericSchemaDocument<rapidjson::GenericValue<rapidjson::UTF16<wchar_t>, rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator> >, rapidjson::CrtAllocator>'
and
runtime error: member access within null pointer of type 'AllocatorType' (aka 'rapidjson::CrtAllocator')