I was able to replicate that the program will crash when the tree view is accessed after putting in text in the 'next' entry, but not creating that entry by accessing it. I have resolved the issue and the program will resume functionality instead of crashing by not attempting to display entries that do not exist yet.
I believe I fixed this issue in the very latest update. If you download the program again the issue should be resolved. I updated this page with information on the fix:
https://actiongameraaron.itch.io/squeaker/devlog/479618/version-0311-fixes-a-cou...
Thank you again for another bug report.