The full error message is something like "Cannot read property 'expParams' of undefined", right? That implies that something is not where it should be.
Experience parameters, a.k.a. "expParams", are part of an actor's class: they encode the experience curve for a class. Check that all actors have a class assigned, or simply delete the spare actors.
If/when you change the actors, I recommend that you clear the starting party afterwards, on the System tab: select the first entry then hold the delete key until you can't scroll down the list. This can help to ensure there are no unintended actors present. Once the list is empty, you can add members back in as needed.