Under what context are you getting this error? It would be supremely helpful to know!
Okay, slightly different from before, actually, now that I checked. Have the available be just Richard, Lester and Janice:
Use all three, navigate to an empty space, then try to "add" someone, and you should see “Type Error: Cannot read property ‘isHidden’ of undefined”
ALSO, making a Formation with less than everyone and trying to add someone who's already added into an empty space will add the first inactive person into the space of the person who was selected?
A.k.a: Party of Lester and Janice, with Richard inactive and trying to add Lester or Janice into the empty space, will add Richard to replace who I was trying to add?
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Okay, I'm assuming ECN is working fine, and I just got really lucky in never seeing any LV 4 or 5 encounters when I was travelling for testing from Lorthil to the Quaysha Caves? What are the average levels of the overworld encounters?