PR #4748 fixes a serious recent blunder

[Edward K. Ream]

PR #4748 fixes a blunder in PR #4740. As a result of this error, opening a new file or a new outline permanently disables tree updates in all outlines!

I have merged the new PR immediately because of the severity of the bug.

Happily, the new code simplifies the old code. There is essentially no chance that the new PR could create new problems. The worst that could happen is that Leo would redraw outlines unnecessarily, but --trace=drawing shows that doesn't happen.

Please continue testing devel.

Edward