Why has Chicago turned into fortress ahead of US Presidential election results?

Houses of former army personnel, huge showrooms, and even restaurants have been covered with plywood ahead of the results of US Presidential elections. Security has been beefed up and police vehicles continue patrolling throughout the day.<br /><br />All this is happening in Chicago but why are such steps being taken?

source https://news.abplive.com/videos/news/world-why-has-chicago-turned-into-fortress-ahead-of-us-presidential-election-results-1381329