Phantom or orphaned false blocks created during interaction with if/else-if/else block

[microbit-robert]

Describe the bug
After adding several else if sections to an if block (at least three), removing them using the - button leaves phantom / orphaned false blocks on the canvas. These can't be interacted with or removed by the user. These also don't appear in the JavaScript.

The following error appears in the developer console:

Uncaught TypeError: Cannot read properties of null (reading 'isShadow')
    at ad.setTargetBlock (main.js:7:230288)
    at ad.setTargetBlock (main.js:7:230304)
    at ad.setStartBlock (main.js:7:230249)
    at ad.handleBlockStart (main.js:7:227938)
    at ep.onMouseDown_ (main.js:7:433042)
    at SVGGElement.o (main.js:7:42156)

The following warning appears in the developer console:

Shadow block of type 'logic_boolean' found at top level. Converting to non-shadow block

To Reproduce
Steps to reproduce the behavior:

1. Open the microbit-phantom-false-blocks.hex file in the following microbit-phantom-false-blocks.zip file
2. Click the + button in the if block to add another else if section
3. Click the - button on the same section just added
4. Note that there is now a 'phantom' false block hidden behind the every block and observe the errors in the developer console
5. This can be repeated subsequent times

Please see the gif below if these steps to reproduce aren't clear.

Expected behavior
Phantom false blocks should not be left behind in this way.

Screenshots

micro:bit version (please complete the following information):
Not hardware related

Desktop (please complete the following information):

• OS: Windows 11
• Browser: Chrome 129.0.6668.90

Additional context
None

[riknoll]

minimal repro steps:

you need to have something attached to the first two condition inputs for the third input to exhibit the bug

[microbit-robert]

It seems highly likely that this is the cause of top-level shadow blocks noted in this separate fix -> microsoft/pxt#10177

[Jaqster]

Related issue - #5899