Importing SB2 Projects with workspace comments converted from block comments

[kchadha]

Expected Behavior

Importing a project into 3.0 should preserve block comments and workspace comments, and comments that were converted from being a block comment to a workspace comment. Saving and loading these projects should also work.

Actual Behavior

Sometimes this is not the case.. see repro below.

Steps to Reproduce

1. Create a new 2.0 project.
2. Drag out two different blocks, e.g. move 10 steps and play sound (for the purposes of distinguishing the blocks)
3. Use the block context menu to add a block comment to one of the blocks, e.g. move 10 steps.
4. Ensure that the project is saved.
5. Move the new block comment away from the block to detach it from the block.
6. Ensure that the project is saved.
   Note: As far as the user can tell at this point, not knowing the history of this comment, this comment looks indistinguishable from a workspace comment. However, in the saved project data, this comment is still represented as connected to the block. Reloading this project in 2.0 itself (not necessary for these repro steps), will re-connect the comment to the block even though that wasn't the stage we saved it in. If you do reload this project such that the comment gets reattached to the block, repeat steps 5 and 6 before proceeding.
7. Delete the original block, e.g. move 10 steps
8. Load this project in 3.0...
9. Observe that there is only a play sound block on the workspace and no comment, workspace or otherwise.
10. Download this project to your computer
11. Upload the project and observe that the project fails to load.

An example of a project that crashes on save/load because of this issue is the Paper Minecraft project by Griffpatch (ID: 10128407)