TypeError: reset(): Argument #1 ($array) must be of type array, null given in reset() (line 341 of docroot/includes/cache.inc)

return ($cache) ? reset($cache) : NULL; issue solved for me

The fix from #2 will only hide the underlying problem, so I would not recommend to use it. The DrupalDatabaseCache::getMultiple() method is supposed to return an array, therefore such fix should not be needed. We would need more informations as per #4. Thanks!