I need more information to reproduce. What exactly were you doing / what was your step by step path to the menu editor (e.g. direct in the Admin Zone, clicking an inline menu edit link, etc)? What did you try editing and to what settings did you attempt?

Just happened when clicking on any existing link, the panel below where you pick an icon was not showing up either. Will check again.

Accessed via clicking the menu edit link from the frontend. Still happening, it's on a branch set to display the children as links (if that helps).

I'm still not able to reproduce. What do you mean by a branch set to display children as links? Also, are you editing the main (top) menu or a custom menu you made somewhere else?

Top menu, and the display children as links is under the sitemap options.

I tried everything I could to trigger the error, but I can't get it to happen. Are you able to open your console log (Inspect element) and then provide a stack trace when you trigger the error?

I do not have the install I reported these potential bugs from anymore, so this is another one we can close for now. Thanks for trying to replicate, I definitely did get that error persistantly.