Saving and loading a project w/more than one comment attached to single block (imported from 2.0) fails

[kchadha]

Expected Behavior

Saving and loading a project should not fail.

Actual Behavior

Saving and loading a project fails if the project in question was imported from a 2.0 project where more than one block comment is attached to a single block.

Steps to Reproduce

1. Create a 2.0 project
2. Drag a block into the project.
3. Right-click the block and click 'add comment'. Write some text in this comment.
4. Repeat step 3 at least 1 more time
5. Save the project
6. Upload the project in 3.0
7. Download the 3.0 project
8. Upload the downloaded 3.0 project and observe project loading failure.