No preview in Attribute form QML dialog

[DelazJ]

What is the bug or the crash?

Vector layer properties --> Form attributes --> Drag-and-drop, and add a QML widget
Switch to a "Rectangle" type, and you'd see on the right a blue rectangle, corresponding to the code in the text box.
Switch now to "Pie chart" or "Bar chart" and you'll notice that, despite the code updated in the text box, nothing happens on the right side

Steps to reproduce the issue

See above.
For information, this is how it used to work (from #7801)

Versions

Issue also already met on an earlier version, with macOS

QGIS version	3.41.0-Master
QGIS code revision	992ccc92939
Libraries
Qt version	5.15.8
Python version	3.11.2
GDAL/OGR version	3.6.2
PROJ version	9.1.1
EPSG Registry database version	v10.076 (2022-08-31)
GEOS version	3.11.1-CAPI-1.17.1
SQLite version	3.40.1
PDAL version	2.6.0
PostgreSQL client version	15.10 (Debian 15.10-0+deb12u1)
SpatiaLite version	5.0.1
QWT version	6.1.4
QScintilla2 version	2.13.3
OS version	Debian GNU/Linux 12 (bookworm)
This copy of QGIS writes debugging output.
Active Python plugins
db_manager	0.1.20
grassprovider	2.12.99
MetaSearch	0.3.6
processing	2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response