If developers miss to define @Init(superclass = true) before derived viewModel class, parent viewModel gets not initialized properly, which typically causes a nasty NPE.
Of course, this is not really a bug, but a very, very big trap! I already spent several hours to debug NPEs from junior developers, who simply missed @Init(superclass = true).
I would suggest to provide a flag in zk.xml which effectively set @Init(superclass = true) as default.