When toggling the value in the UI we simply overwrite the value in UserConfig;
this would be bad if there was ever a chance that we want to write the user
config back to disk, but it is very unlikely that we can do that, because
currently we have no way to tell which parts of the config come from the global
config file and which ones come from a repo-local one.
The option doesn't have any affect in these views, so we don't need to toggle it
here. But the problem was the HandleFocus call at the end: this would activate
the wrong view, so we need to avoid it here.
Show an error if the user tries to turn the option on, to let them know that it
doesn't work here.
