If the EU have got any sense of comedy they should give us an extension to December 31st 2020.   
If we ratify departure we leave the next day, no problem: whether that is tomorrow, the end of the month or some time next spring. So if needed time for an election that isn't held over Christmas or in January.   
However I think they will sit tight for now and see if the legislation can get through the HOC by the end of the month. The numbers suggest  that this is possible now. If not they will let us roll on.   
I know some people still don't get it, but the one thing that ain't ever going to happen is no deal.