Dragging block into toolbox causes error

[sappm01]

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Description

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Dragging block into toolbox causes error:
Ben: another excellent find. From the stack trace, this seems also somehow tied back to the undo case found above. I suspect they have a similar root cause and fix.

Reproduction steps

Stack trace

Screenshots

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Browsers

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[maribethb]

we should at least root cause this to make sure it's not a v12 problem. if it's not, it can be pushed to feature complete

[rachel-fenichel]

Possibly fixed by #8981 if this was the same issue as #8971

[RoboErikG]

I think this was fixed as I don't see any errors with dragging blocks to the toolbox. Closing but let me know if I misunderstood the issue!