The issue was that the new switch code to support stacked actors sends
`'activate` to every actor at a given grid coordinate regardless of type. This
is fine for most uses, but because this level had an emitter over a wall, the
switch tries to `'activate` the wall. Replacing the wall tile with a floor
tile (which produces no actor and therefore receives no `'activate` message)
works just fine. Instead of adding type checking to allow a hacky and needless
use case, I instead advise just not sticking switch targets over walls or other
actors that don't support `'activate`.
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