I suspect your entire event 93 runs on every tick. When boss3energy<=0, this event will repeat 60 times per second until the boss sprite is destroyed or layout is changed. This doesn't explain why it crashes the game, but it's still quite bad - running this event repeatedly will create lots of delayed "wait" threads, will play multiple overlapping sounds etc. You should make sure that this event runs only once.
You can add "System Trigger Once" condition to this event, but only if you have just one instance of the boss sprite on the layout. Be very careful with this condition, it's very often used incorrectly and may cause very weird bugs.
You don't need to unload images when you switch layouts, the engine will do it for you.
Using "Wait 20 seconds" is also a bad practice. This is a very long delay and you don't have control over it - you can't cancel or pause it. I suggest using Timer behavior instead.