I am checking in a partial hot fix for this for 26.0.1, so that should be part of an upcoming cumulative hot fix.
A secondary problem was that the prototype "bool startupSequence() override" was not being tagged as a virtual function, even though in C++ you are only allowed to override virtual functions, so it is fair to assume it is virtual despite the missing qualification. This tagging issue will be fixed in 26.0.2 or 27.0.0, whichever comes first. A workaround is to put in the virtual keyword.