Fix dumb demo.html bugs intro'd by api.non-plugin

https://github.com/mathquill/mathquill/commit/f4ce7e9647cb2d3d05ff47e0551a7ed7ee258057#commitcomment-5382849
and
https://github.com/mathquill/mathquill/commit/f4ce7e9647cb2d3d05ff47e0551a7ed7ee258057#commitcomment-5382864