maybeFixupViewportPosition: Uncaught TypeError: document.getElementsByClassName(...)[0] is undefined

[jyn514]

docs.rs/templates/rustdoc/body.html

Line 9 in 93a49e5

const navBarHeight = document.getElementsByClassName("nav-container-rustdoc")[0].offsetHeight;

This was first added in #873; I think it regressed in #1089.

cc @Nemo157 @antifuchs - does one of you have time to take a look?