List monitors throw errors

[SheepTester]

Ref: https://scratch.mit.edu/users/Sheep_maker/#comments-159747126

HTMLifying project 529745315 with default settings throws the following error in the console when you click on

TypeError: Cannot set properties of undefined (setting 'textContent')
    at about:srcdoc:1301:41
    at Array.forEach (<anonymous>)
    at about:srcdoc:1282:19
    at about:srcdoc:447:49138
    at cB.__iterate (about:srcdoc:447:37565)
    at RB.__iterate (about:srcdoc:447:49106)
    at RB.forEach (about:srcdoc:447:61531)
    at g.<anonymous> (about:srcdoc:1196:14)
    at g.w.emit (about:srcdoc:422:29476)
    at g.value (about:srcdoc:447:491652)