"No basemap layer selected" error when theme is selected

[spono]

Hi,
trying to pack up a minimal project with the same settings and -partly- data of #587 , now I get an error saying "No basemap layer selected. Check configuration"...even tough basemap is set to use a map theme which includes two layers: hillshade and contour lines. In this case, nothing changes if layers are within the "RASTER BASE" group (see image attached in 587). I tested also with a new profile but it the same.

Here the minimum reproducible project.

I'm working on W10 with QGIS updated few days ago as follows:

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

QGIS version	3.34.8-Prizren	QGIS code revision	91642333
Qt version	5.15.13
Python version	3.12.4
Compiled against GDAL/OGR	3.9.0	Running against GDAL/OGR	3.9.1
PROJ version	9.4.0
EPSG Registry database version	v11.004 (2024-02-24)
GEOS version	3.12.2-CAPI-1.18.2
SQLite version	3.45.1
PDAL version	2.6.3
PostgreSQL client version	16.2
SpatiaLite version	5.1.0
QWT version	6.2.0
QScintilla2 version	2.14.1
OS version	Windows 10 Version 2009
Active Python plugins
qfieldsync	v4.9.1
QuickOSM	2.2.3
db_manager	0.1.20
MetaSearch	0.3.6
processing	2.12.99

QGIS version 3.34.8-Prizren QGIS code revision [91642333](https://github.com/qgis/QGIS/commit/91642333) Qt version 5.15.13 Python version 3.12.4 Compiled against GDAL/OGR 3.9.0 Running against GDAL/OGR 3.9.1 PROJ version 9.4.0 EPSG Registry database version v11.004 (2024-02-24) GEOS version 3.12.2-CAPI-1.18.2 SQLite version 3.45.1 PDAL version 2.6.3 PostgreSQL client version 16.2 SpatiaLite version 5.1.0 QWT version 6.2.0 QScintilla2 version 2.14.1 OS version Windows 10 Version 2009

Active Python plugins
qfieldsync
v4.9.1
QuickOSM
2.2.3
db_manager
0.1.20
MetaSearch
0.3.6
processing
2.12.99