The problem that the OP reports with the scroll is really a problem with the non-visible elements, which are not displayed even if the "Display hidden components in Viewer" option is activated. Since these elements are not displayed, there is nothing to scroll....when the user marks them as visible and they are visible, the user can scroll without problems. So it really seems like a problem with the "Display hidden components in Viewer" option.
Now, are you experiencing any other type of problems with the Palette? what do you mean with "manually scroll"?
A singleton is a class that should only ever have one instance. The code should never try to create a second one, and clearly in this situation it is.
However, this check is to prevent a second copy of the checkbox from ever being created. After this error is logged, the code returns the existing checkbox instead of creating a new copy.
I need to figure out what circumstances are causing App Inventor to try to create a second copy of the hidden components checkbox, but that error in and of itself is not causing your problem. It may be indicative of why you're seeing this problem and we are not.
In case it's useful...in my case I have observed that the problem arises when I switch between projects. The first project I open has no problems, but if I change to other projects the Hidden Components option does not work. I have to refresh and reopen the project (as if it were the first) for it to work. If I change the project again, it stops working for the other projects (I also see the red letter message when opening projects.).
The preview server I set up is behind App Inventor's master branch, so it doesn't have the UI update code present. It's intent is solely to allow people to test compiled apps against SDK 34.