Not possible to drag multi-block assemblies from toolbox in javafx

[KlemenDEV]

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Description

Following up #7444, now rendering works fine without glitches. However, now when a block with sub-blocks is dragged from the toolbox, an error is thrown and Blockly becomes unusable.

Does not happen on eg. Chrome, just JavaFX.

Reproduction steps

Reproducible example: demo.zip

Used the same way as in #7444, except now try dragging block assembly from toolbox and see error shown in terminal. Does not happen if a single block is in the toolbox (eg. just print, not print with text block attached).

Stack trace

The crash happens because svgGroup is null at

blockly/core/renderers/common/drawer.ts

Line 319 in ca4fa7e

svgGroup.setAttribute('display', 'none');

There could be more to it, this is just what I have managed to debug with limited JavaFX webview debug capabilites

[rachel-fenichel]

If you can track this down to a bug that we can fix by adding another IS_JAVA_FX flag we will consider it, but we don't have the resources to put a ton of time into debugging what looks like implementation issues in JavaFX itself.