Problem navigating past text category and back

[microbit-matt-hillsdon]

Using the demo built against v12 navigate to the toolbox and use the arrows to navigate to "Lists" then back up to "Text".

The console log is suddenly spammed with "Ignoring non-existant field name in block text_append" and performance suffers. I could not go right into the flyout from this state.

toolbox-nav-bug.mov

I used https://insert-connection.blockly-keyboard-experimentation.pages.dev/ which has #510 for this test. I doubt it can be that fix that's caused the issue.

[microbit-matt-hillsdon]

(original video was incorrect, edited to replace)

[rachel-fenichel]

Fixed by #9025