You should first see if your app works with getState on api.steemitdev.com. If it does, you're good. If not, this is how to switch if you were using dsteem:

https://developers.steem.io/tutorials-javascript/get_state_replacement_api

But the general point is, if you use their getState(‘witnesses’), use databaseAPI.call('get_witnesses_by_vote', ['', 10]) instead.