projecting label coordinates for multiPoint geometry crashes QGIS

[tschuettenberg]

What is the bug or the crash?

When using the expression functions "x" / "y" in combination with the function "project(point,distance,azimuth[,elevation])" on a multiPoint geometry in order to offset the label's position, QGIS crashes.

Steps to reproduce the issue

1. find a geopackage "label_issue.gpkg" (500 KB) with data, qml and project file here: https://ablage.qgis.de/index.php/s/ayAYG4js6xmi3BT
2. open the project, there are two layers of identical points: "ap_test_point" with geometry type (single) point and styling and "ap_test_multipoint" of geometry type multiPoint, although there are no multipart objects, every feature consists of only one single geometry/part.
3. on the layer ap_test_multipoint set a label with data defined override for the placement of the label coordinates using x and y of the projected geometry:
   x ( project ( $geometry, 5, 3.14 )) remember: $geometry is multiPoint here, this works fine with single point geometries.
4. typing or pasting this expression crahes QGIS immediately.
   (The expression on "ap_test_point" looks like this x(project($geometry,min(@map_scale*0.04,5), -"S0P0P_SY_ROTATION")) and would also crash QGIS on the multipoint layer.)

Versions

QGIS version
3.16.11-Hannover
QGIS code revision
26cc1c7
Compiled against Qt
5.11.2
Running against Qt
5.11.2
Compiled against GDAL/OGR
3.1.4
Running against GDAL/OGR
3.1.4
Compiled against GEOS
3.8.1-CAPI-1.13.3
Running against GEOS
3.8.1-CAPI-1.13.3
Compiled against SQLite
3.29.0
Running against SQLite
3.29.0
PostgreSQL Client Version
11.5
SpatiaLite Version
4.3.0
QWT Version
6.1.3
QScintilla2 Version
2.10.8
Compiled against PROJ
6.3.2
Running against PROJ
Rel. 6.3.2, May 1st, 2020
OS Version
Windows 7 SP 1 (6.1)
Active python plugins
alkisplugin;
BaSYSPlanApp;
changeDataSource;
DataDrivenInputMask;
DataPlotly;
DbgBaum;
dimensioning;
Discovery;
EasyTemplatePrint;
instantprint;
maplibrary;
menu_from_project;
pg_metadata;
PostNAS_Search;
qgiscloud;
xplanung;
db_manager

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

The given points here represent the connection of sewer pipes, so it does not make sense to be multi. The pipe connections are normaly reperested by the single point where their center lines meet. My solution/workaround therefore is to avoid a useless conversion to multipoint. Having these converted to multipoint was by accident anyway, but reveald the described issue.

I'm mot 100% sure, if the used expression must work on multipoints but none the less is the crash not the best way to let the user know...

Cheers!