Support for the Ruby 2.3 series has ended. See here for reference.
Tabset
# File tk/lib/tkextlib/blt/tabnotebook.rb, line 77 def get_tab(index) if (idx = tk_send_without_enc('id', tagindex(index))).empty? nil else Tk::BLT::Tabset::Tab.id2obj(self, idx) end end