Replenish Board
replenish add-on replace all null values in Board into valid item.
It's also included in other add-ons (i.e. removeLine). So you don't have to do it manually.
If you want to do it manually --
import { replenish } from '<Path to BoardPlugin>';
// then whenever needed
replenish() // finds empty posiiton in board and replace with new.
/**
* It's best to use it after handling falling of items,
* otherwise it'll most likely throw error.
**/