Fix fatal exception related to Hide tone-change notation

This was reported:

The wiki Zi: pages are replaced by errors of this type (from Zi:人):

[YBfk5idD--rqdXz2PcrNkgAAAMA] 2021-02-01 11:24:22: Fatal exception of type "Error"

(tested on macOS Safari, Linux Firefox, Android Chrome)

After messing with the preferences, it looks like it's caused by enabling the "Hide tone-change notation" setting.