Fails to build

In Ubuntu plucky, and also debian, massxpert2 8.5.0 is failing to build:

cd /home/ubuntu/git/packages/massxpert2/massxpert2/debian/build/src && /usr/bin/c++ -DQT_CORE_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_NO_DEBUG -DQT_NO_DEBUG_OUTPUT -DQT_PRINTSUPPORT_LIB -DQT_SVGWIDGETS_LIB -DQT_SVG_LIB -DQT_WIDGETS_LIB -DQT_XML_LIB -I/home/ubuntu/git/packages/massxpert2/massxpert2/debian/build/src -I/home/ubuntu/git/packages/massxpert2/massxpert2/src -I/home/ubuntu/git/packages/massxpert2/massxpert2/debian/build/src/massxpert2_autogen/include -I/home/ubuntu/git/packages/massxpert2/massxpert2/debian/build -I/home/ubuntu/git/packages/massxpert2/massxpert2 -I/home/ubuntu/git/packages/massxpert2/massxpert2/src/nongui -I/home/ubuntu/git/packages/massxpert2/massxpert2/src/gui -isystem /usr/include/pappsomspp -isystem /usr/include/IsoSpec++ -isystem /usr/include/x86_64-linux-gnu/qt6/QtCore -isystem /usr/include/x86_64-linux-gnu/qt6 -isystem /usr/lib/x86_64-linux-gnu/qt6/mkspecs/linux-g++ -isystem /usr/include/x86_64-linux-gnu/qt6/QtXml -isystem /usr/include/x86_64-linux-gnu/qt6/QtNetwork -isystem /usr/include/x86_64-linux-gnu/qt6/QtGui -isystem /usr/include/x86_64-linux-gnu/qt6/QtWidgets -isystem /usr/include/x86_64-linux-gnu/qt6/QtPrintSupport -isystem /usr/include/x86_64-linux-gnu/qt6/QtSvg -isystem /usr/include/x86_64-linux-gnu/qt6/QtSvgWidgets -g -O3 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -ffile-prefix-map=/home/ubuntu/git/packages/massxpert2/massxpert2=. -flto=auto -ffat-lto-objects -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -fdebug-prefix-map=/home/ubuntu/git/packages/massxpert2/massxpert2=/usr/src/massxpert2-8.5.0-1 -Wdate-time -D_FORTIFY_SOURCE=3 -O3 -DNDEBUG -std=gnu++17 -Wno-unknown-pragmas -Wall -pedantic -Wextra -MD -MT src/CMakeFiles/massxpert2.dir/gui/ProgramWindow.cpp.o -MF CMakeFiles/massxpert2.dir/gui/ProgramWindow.cpp.o.d -o CMakeFiles/massxpert2.dir/gui/ProgramWindow.cpp.o -c /home/ubuntu/git/packages/massxpert2/massxpert2/src/gui/ProgramWindow.cpp
In file included from /usr/include/x86_64-linux-gnu/qt6/QtWidgets/qwidget.h:9,
                 from /usr/include/x86_64-linux-gnu/qt6/QtWidgets/qdialog.h:8,
                 from /usr/include/x86_64-linux-gnu/qt6/QtWidgets/QDialog:1,
                 from /usr/include/libXpertMassGui/MassDataClientServerConfigDlg.hpp:38,
                 from /home/ubuntu/git/packages/massxpert2/massxpert2/src/gui/ProgramWindow.cpp:35:
/usr/include/x86_64-linux-gnu/qt6/QtCore/qobject.h: In instantiation of ‘QList<T> QObject::findChildren(QAnyStringView, Qt::FindChildOptions) const [with T = MsXpS::massxpert::AbstractMainTaskWindow*; Qt::FindChildOptions = QFlags<Qt::FindChildOption>]’:
/usr/include/x86_64-linux-gnu/qt6/QtCore/qobject.h:184:31: required from ‘QList<T> QObject::findChildren(Qt::FindChildOptions) const [with T = MsXpS::massxpert::AbstractMainTaskWindow*; Qt::FindChildOptions = QFlags<Qt::FindChildOption>]’
  184 | return findChildren<T>(QAnyStringView{}, options);
      | ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/ubuntu/git/packages/massxpert2/massxpert2/src/gui/ProgramWindow.cpp:177:74: required from here
  177 | dynamic_cast<QObject *>(this)->findChildren<AbstractMainTaskWindow *>();
      | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/usr/include/x86_64-linux-gnu/qt6/QtCore/qobject.h:167:62: error: static assertion failed: No Q_OBJECT in the class passed to QObject::findChildren
  167 | static_assert(QtPrivate::HasQ_OBJECT_Macro<ObjType>::Value,
      | ^~~~~
/usr/include/x86_64-linux-gnu/qt6/QtCore/qobject.h:167:62: note: ‘QtPrivate::HasQ_OBJECT_Macro<MsXpS::massxpert::AbstractMainTaskWindow>::Value’ evaluates to false
make[4]: *** [src/CMakeFiles/massxpert2.dir/build.make:985: src/CMakeFiles/massxpert2.dir/gui/ProgramWindow.cpp.o] Error 1
make[4]: Leaving directory '/home/ubuntu/git/packages/massxpert2/massxpert2/debian/build'
make[3]: *** [CMakeFiles/Makefile2:182: src/CMakeFiles/massxpert2.dir/all] Error 2
make[3]: Leaving directory '/home/ubuntu/git/packages/massxpert2/massxpert2/debian/build'
make[2]: *** [Makefile:159: all] Error 2
make[2]: Leaving directory '/home/ubuntu/git/packages/massxpert2/massxpert2/debian/build'
make[1]: *** [debian/rules:43: override_dh_auto_build] Error 2
make[1]: Leaving directory '/home/ubuntu/git/packages/massxpert2/massxpert2'
make: *** [debian/rules:23: build] Error 2
dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2

Debian bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1084295

Ubuntu bug report: https://bugs.launchpad.net/ubuntu/+source/massxpert2/+bug/2091412