This module has been broken for a while in core.
- Uncaught TypeError: Cannot call method 'join' of null
This is because wgSearchNamespaces is also broken (not output the same way the module is loaded, so by default the module is loaded but wgSearchNamespaces is not set).
It also seems to interfere still with Vector's SimpleSearch (in that both modules are loaded, though they don't initialize both, at least that's been fixed since)
We don't see the exception on Wikimedia wikis because the Vector extension overrides mwsuggest and prevents it from reaching the error.