Appreciation for using the nightly build and reporting it 
I'll take a look. There must be a regression in arduino/arduino-ide#1847 after arduino/arduino-ide#2144.
Please open a GitHub issue when you see such a regression. I monitor that channel more often. Thank you! Your previous input (on Nightly build 0705 wants to update to 2.1.1) was also super helpful.
Update:
Very close. IDE2 does not refresh menus as often as it used to, and it's a coincidence that the New Tab handler ID will be 0, which is falsy in JavaScript. See some additional findings here.
Update 2:
The fix is on the way: fix: falsy context menu `handlerId` by kittaakos · Pull Request #2183 · arduino/arduino-ide · GitHub. Thank you for reporting it 