im going to sticky this one because its going to come up every time old saves break an update
there's several ways you can get around this, repairing your save by hand, rescuing your party or restarting fresh.
the first method involves looking at the NPC.dat and Registry.dat in your save folder. that is located in folder the launcher is in/pokemon/saves/ with a folder for each character. you will have to look at the maps that changed and figure out whats missing from your save to trigger the event, usually its a removed npc, a registry flag that hasnt been set etc. you can reset events and trainers by removing them from the registry.
the second method is easier, you go to your character's folder and copy it somewhere else. then you start a new game and play through to where you were. then you can go into your backup of your save and find party.dat and box.dat, you can now either copy the file over your new save's one to get all your pokemon back, or you can merge them. each pokemon is a line in the files so to have that pokemon again with those stats and moves you just copy/paste their line into the new save's file.
the third method is the nuclear option, restart the game. honestly you aren't losing much at this point, and next update will maybe have new starters again so don't get too attached to your saves. but at least there are ways of repairing a save or at least preserving your pokemon