bug: only the last ndv window still has the data

[tlambert03]

another bug this PR reveals: only the last ndv window still has the data. changing things like contrast limits, or current slice on older windows doesn't do anything. So we need to review ownership of the data. Also: it fails silently, so we should try to figure out where/why it fails and surface some sort of message

Originally posted by @tlambert03 in #51 (comment)