Comments in imported project can be removed using undo

[mfrtrifork]

Expected Behavior

The history of the project should be cleared and "Undo" button should be disabled.

Actual Behavior

"Undo" button is enabled and removes the comment (moves one comment at a time which is connected to a block).

Steps to Reproduce

1. Create a project with comments.
2. Save to sb3 file
3. Open file.
4. Right click and see that undo is enabled.

Operating System and Browser

Mac OS 10.14.4
75.0.3770.142 (Official Build) (64-bit)

[DNin01]

I think Scratch is loading the sprite's content without comments that are attached to blocks and then adding the attached comments afterward which gets recorded in undo history. But when that happens, the content doesn't get saved in undo/redo history, so undoing and then redoing inserts a blank comment.