@author: no shade for not continuing to update the mod, btw. Just wanted to share a way to fix a crash for people to do themselves.

Steps:

1. Go to your Factorio "mods" folder
2. Unzip "attach-notes_0.5.1.zip"
3. In the unzipped folder, open "/scripts/entity-notes/controller.lua" with Notepad or another text editor
4. Change line 176 from

elseif cache.openedEntityGui and cache.openedEntityGui.unit_number == unitNumber then

to

elseif cache.openedEntityGui and cache.openedEntityGui.valid and cache.openedEntityGui.unit_number == unitNumber then

Basically, add and cache.openedEntityGui.valid to the middle of the line.

Lua isn't my thing, so I have no idea if this'll break something else down the line, but it stops the crash so shrug