Comments attached to variable blocks are altogether lost upon saving

### Expected Behavior

All comments are saved/restored when a project is saved/loaded.

### Actual Behavior

Comments attached to variable blocks disappear.

### Steps to Reproduce

Create a project that contains a variable block and attach a comment to it. Save it, then load it. Note that the comment is gone.

### Operating System and Browser

Firefox 65.0b7, Linux (Debian).
