We noticed this issue with the 0.2.x release of snide/sphinx_rtd_theme. Because
we are calling toctree unconditionally, we notice a bug in the singlehtml
builder when the docs have an empty/nonexistant toctree. In this case,
`fix_refuris` as being passed `None`, which failed to traverse, throwing an
exception.
This conditionally fixes the refuris instead.