[Mesh] Dataset groups in symbology tab are not synced with Source tab until you reopen properties dialog

[DelazJ]

What is the bug or the crash?

If you remove/rename a dataset group under the Source tab, and press Apply, the changes do not reflect in available dataset groups list under Symbology-->Datasets tab

Steps to reproduce the issue

1. Open properties dialog of a mesh layer
2. Under Source tab, rename/uncheck some of the dataset groups
3. Click Apply
4. Switch to Symbology --> Datasets tab. The dataset groups list is still as if you did no changes
5. Reopen the Properties dialog. The dataset groups tree in "Datasets" tab is now synced with what you set in the Source tab

Versions

<style type="text/css"> p, li { white-space: pre-wrap; } </style>

QGIS version	3.21.0-Master	QGIS code revision	8b2a336
Qt version	5.12.8
Python version	3.8.10
GDAL/OGR version	3.0.4
PROJ version	6.3.1
EPSG Registry database version	v9.8.6 (2020-01-22)
Compiled against GEOS	3.8.0-CAPI-1.13.1	Running against GEOS	3.8.0-CAPI-1.13.1
SQLite version	3.31.1
PostgreSQL client version	12.8 (Ubuntu 12.8-0ubuntu0.20.04.1)
SpatiaLite version	4.3.0a
QWT version	6.1.4
QScintilla2 version	2.11.2
OS version	Ubuntu 20.04.3 LTS
Active Python plugins	trackable_project_files closest_points MapsPrinter MemoryLayerSaver plugin_reloader layertreeicons db_manager grassprovider MetaSearch processing sagaprovider

Additional context

No response