Problem editing index.html file

I'm using Delphi 10.4.2 with WEB Core. Normally everything works fine.

When I open the index.html file in the project to edit it, it opens ok. But when I try to save it or close the file, I get an IDE error that says "List index out of bounds(-1)". I know what that means, but it's somewhere in the IDE or some other package that's loaded in the IDE. I cannot do anything except kill Delphi.

When opening the project directly in Delphi, it shows an exception: "Cannot locate menu item 'InstantBuilderItem'". I have no idea what that might be.

How in the heck can I open this project without displaying any of the files, or at least THIS file (index.html)? It's the only file that this happens with.

I have never seen such error nor got reports from it.
You have other Delphi IDE plugins installed that might somehow interfere?
Also, if this is only happening to one HTML file, what is particular about it?

Just an idea, no clue whether it will help or not. But when I've had Delphi projects that were decidedly uncooperative in this manner, I'd delete all the .dsk files in the folder. I think (?) that's where it stores what tabs are open. Or something. No harm as they'll get regenerated. Just something to try if you're stuck. No clue about the underlying problem and never run across that error though.

Ahh, the .dsk file. I forgot about that. I've had this happen while trying to edit any kind of .html file inside of the IDE for as long as I can remember, although it's not something I do much b/c pascal and html don't usually swim in the same waters. But with WEB Core, we need to edit those files from time to time. If I double-click to open an html file from the Project Manager, I'm guessing there's something loaded in the IDE that just gags on it. It's easy to edit them in the text editor, so that's what I usually do.

Odd. I've had the occasional hiccup with other files, but never HTML files. Or CSS files. At least that I can recall. There's really not much for the IDE to deal with there, so not sure why it is getting tripped up.

It's probably something I've got loaded into the IDE. I don't feel like spending an hour or so tracking it down. I was hoping maybe someone else might have seen this as well.