Error in bootstrap.js when `back_to_top_button` is configured to `False`

[dstansby]

Over in Matplotlib we set "back_to_top_button": False, in the theme config options, but this is causing an error in bootstrap.js on this line because it means the back-to-top element does not exist.

This error can be seen currently on https://matplotlib.org/, and I can reproduce with a local HTML build of our website, which gives a traceback to the line above.