fix: Fix exception when disposing of a workspace with a variable block obscuring a shadow block.

[gonfunko]

The basics

• I validated my changes

The details

Resolves

Fixes gonfunko/scratch-blocks#210

Proposed Changes

This PR fixes a tricky bug related to workspace disposal. When a WorkspaceSvg instance A is disposed:

1. It sets this.rendered = false
2. It disposes of its flyout
3. The flyout disposes of its flyout workspace B
4. The flyout workspace B, as part of disposing itself, clears its variable map, which is the same as the variable map of workspace A, because when a flyout is created the parent workspace shares its variable map with the flyout
5. When the variable map clears itself, it calls VariableMap.deleteVariable for each variable it contains
6. VariableMap.deleteVariable calls VariableMap.getVariableUsesById to identify references to the variable
7. VariableMap.getVariableUsesById calls Variables.getVariableUsesById, passing its workspace, which, because this variable map was shared with the flyout by Workspace A, is Workspace A, which is in the process of being torn down
8. Variables.getVariableUsesById retrieves the blocks of Workspace A, which in this (mildly) contrived example includes a variable block obscuring a shadow block
9. VariableMap.deleteVariable calls dispose on the variable block obscuring a shadow block on Workspace A, in order to clean up references to the being-deleted variable
10. Because the variable block was obscuring a shadow block, the connections attempt to create a new shadow block to restore it
11. When the respawned shadow block gets created on workspace A, which is being disposed, the BlockSvg constructor checks to ensure that its workspace is rendered (to prevent creating a BlockSvg on a regular Workspace), but because the workspace is being disposed, rendered is false, so it throws
12. @gonfunko spends the better part of two days tracking this down

This PR only clears the variable map when the workspace being disposed is not a flyout workspace, which resolves the issue.

[rachel-fenichel]

This is a wonderfully simple fix for a complex problem. Thanks for the write-up.