Unhandled Error: Error: Tried to serialize a variable field with no variable.

[fsih]

Expected Behavior

No oops

Actual Behavior

Oops

Steps to Reproduce

1. Go to https://llk.github.io/scratch-gui/develop/#221549537
2. Click on the fourth sprite
3. Click on the third sprite

Now there are some errors:
raven.js:429 Uncaught Error: Variable "Screen" is already in use and its id is "U)^l%k)ew%F=LoUL`jPI-Screen" which conflicts with the passed in id, "cDw3AqT_3chH3kWRAL(I-Screen".
at Blockly.VariableMap.createVariable (blockly_compressed_vertical.js?./node_modules/imports-loader?this=>window!./node_modules/exports-loader?Blockly&goog:274)

4. Change tabs
5. Change back to blocks

Now there are more errors, one of which is fatal:
gui Unhandled Error: Error: Tried to serialize a variable field with no variable.
at Object.Blockly.Xml.fieldToDomVariable_ (webpack://ScratchBlocks/./blockly_compressed_vertical.js?./node_modules/imports-loader?this=%3Ewindow!./node_modules/exports-loader?Blockly&goog:827:128)

The original project was created in scratch 2 by putting a few sprites from a different project into the backpack, and then putting the backpacked sprites into a new project.
This was the original project: https://scratch.mit.edu/projects/218507424/

Operating System and Browser

Windows 10 Chrome

[paulkaplan]

It would be really good to have a full repro case of this from end-to-end @BryceLTaylor @fsih

[fsih]

@kchadha did a test and said she knew what was going on, and it would probably be fixed when per-sprite monitors are implemented. Both the third and fourth sprite referred to the same global variable in the original project. When they are both pulled into the backpack and into a new project, they both try to create the same global variables when you load the workspace.

[paulkaplan]

I believe this has the same root cause as #2180, that is, having multiple sprite-local variables with the same names, but I'll leave them both open because this is another repro case.