If one of these things happen it might happen in 2020 or possibly sooner.
One way violence could break out is, if the President is removed from office based on lies and false evidence in a rigged trial in the Senate.
Another way that violence might be triggered is, if Trump remains in office and he is re-elected in a landslide and demoncrats losing a boatload of other offices. The left will then have no other option but violence. I think this is the most likely scenario. The left has already shown that they are far more likely to commit violence that conservatives.