SUMMARY
(gdb) bt
#0  Marble::GeoSceneTileDataset::tileSize (this=0x0) at /var/tmp/portage/kde-apps/marble-9999/work/marble-9999/src/lib/marble/geodata/scene/GeoSceneTileDataset.cpp:183
#1  0x00007ffff7ccac6c in Marble::MergedLayerDecorator::tileSize (this=<optimized out>) at /var/tmp/portage/kde-apps/marble-9999/work/marble-9999/src/lib/marble/MergedLayerDecorator.cpp:159
#2  0x00007ffff7b58347 in Marble::TextureLayer::tileSize (this=this@entry=0x5555559b5760) at /var/tmp/portage/kde-apps/marble-9999/work/marble-9999/src/lib/marble/layers/TextureLayer.cpp:498
#3  0x00007ffff7b7ba8e in Marble::DownloadRegionPrivate::rad2PixelX (this=this@entry=0x555558cdaf40, lon=0.64133849577061264, textureLayer=textureLayer@entry=0x5555559b5760)
    at /var/tmp/portage/kde-apps/marble-9999/work/marble-9999/src/lib/marble/DownloadRegion.cpp:50
#4  0x00007ffff7b7bd9c in Marble::DownloadRegion::region (this=0x7fffe000b3b8, textureLayer=0x5555559b5760, downloadRegion=...) at /var/tmp/portage/kde-apps/marble-9999/work/marble-9999/src/lib/marble/DownloadRegion.cpp:104
#5  0x00007ffff7b7ec21 in Marble::DownloadRegionDialog::region (this=this@entry=0x55555742f100) at /var/tmp/portage/kde-apps/marble-9999/work/marble-9999/src/lib/marble/DownloadRegionDialog.cpp:339
#6  0x00007ffff7b7ee87 in Marble::DownloadRegionDialog::updateTilesCount (this=0x55555742f100) at /var/tmp/portage/kde-apps/marble-9999/work/marble-9999/src/lib/marble/DownloadRegionDialog.cpp:428
#7  0x00007ffff7b7fbf4 in Marble::DownloadRegionDialog::qt_static_metacall (_o=<optimized out>, _c=<optimized out>, _id=<optimized out>, _a=<optimized out>)
    at /var/tmp/portage/kde-apps/marble-9999/work/marble-9999_build/src/lib/marble/marblewidget_autogen/include/moc_DownloadRegionDialog.cpp:116
#8  0x00007ffff671f6c8 in doActivate<false> (sender=0x55555793ff60, signal_index=8, argv=0x7fffffffce80) at /var/tmp/portage/dev-qt/qtcore-5.15.2-r2/work/qtbase-everywhere-src-5.15.2/src/corelib/kernel/qobject.cpp:3898
#9  0x00007ffff671969f in QMetaObject::activate (sender=<optimized out>, m=m@entry=0x7ffff7f839e0 <Marble::TileLevelRangeWidget::staticMetaObject>, local_signal_index=local_signal_index@entry=1, argv=argv@entry=0x7fffffffce80)
    at /var/tmp/portage/dev-qt/qtcore-5.15.2-r2/work/qtbase-everywhere-src-5.15.2/src/corelib/kernel/qobject.cpp:3946
#10 0x00007ffff7c09ad6 in Marble::TileLevelRangeWidget::bottomLevelChanged (this=<optimized out>, _t1=<optimized out>)
    at /var/tmp/portage/kde-apps/marble-9999/work/marble-9999_build/src/lib/marble/marblewidget_autogen/include/moc_TileLevelRangeWidget.cpp:163
#11 0x00007ffff7c09dab in Marble::TileLevelRangeWidget::qt_static_metacall (_o=<optimized out>, _c=<optimized out>, _id=<optimized out>, _a=<optimized out>)
    at /var/tmp/portage/kde-apps/marble-9999/work/marble-9999_build/src/lib/marble/marblewidget_autogen/include/moc_TileLevelRangeWidget.cpp:88
#12 0x00007ffff671f6c8 in doActivate<false> (sender=0x555558892e50, signal_index=8, argv=0x7fffffffcfb0) at /var/tmp/portage/dev-qt/qtcore-5.15.2-r2/work/qtbase-everywhere-src-5.15.2/src/corelib/kernel/qobject.cpp:3898
#13 0x00007ffff734b04e in QSpinBox::valueChanged(int) () from /usr/lib64/libQt5Widgets.so.5
#14 0x00007ffff7348019 in QAbstractSpinBoxPrivate::setValue(QVariant const&, EmitPolicy, bool) () from /usr/lib64/libQt5Widgets.so.5
#15 0x00007ffff7348d2d in QAbstractSpinBox::stepBy(int) () from /usr/lib64/libQt5Widgets.so.5
#16 0x00007ffff73468ff in QAbstractSpinBoxPrivate::updateState(bool, bool) () from /usr/lib64/libQt5Widgets.so.5
#17 0x00007ffff71ea2f8 in QWidget::event(QEvent*) () from /usr/lib64/libQt5Widgets.so.5
#18 0x00007ffff71aa0ff in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib64/libQt5Widgets.so.5
#19 0x00007ffff71b0f83 in QApplication::notify(QObject*, QEvent*) () from /usr/lib64/libQt5Widgets.so.5
#20 0x00007ffff66eb667 in QCoreApplication::notifyInternal2 (receiver=0x555558892e50, event=0x7fffffffd690) at /var/tmp/portage/dev-qt/qtcore-5.15.2-r2/work/qtbase-everywhere-src-5.15.2/src/corelib/kernel/qcoreapplication.cpp:1063
#21 0x00007ffff71aff99 in QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool, bool) () from /usr/lib64/libQt5Widgets.so.5
#22 0x00007ffff72021ab in QWidgetWindow::handleMouseEvent(QMouseEvent*) () from /usr/lib64/libQt5Widgets.so.5
#23 0x00007ffff72054ae in QWidgetWindow::event(QEvent*) () from /usr/lib64/libQt5Widgets.so.5
#24 0x00007ffff71aa0ff in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib64/libQt5Widgets.so.5
#25 0x00007ffff66eb667 in QCoreApplication::notifyInternal2 (receiver=0x5555566bb370, event=0x7fffffffd940) at /var/tmp/portage/dev-qt/qtcore-5.15.2-r2/work/qtbase-everywhere-src-5.15.2/src/corelib/kernel/qcoreapplication.cpp:1063
#26 0x00007ffff6b79e28 in QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent*) () from /usr/lib64/libQt5Gui.so.5
#27 0x00007ffff6b7b665 in QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent*) () from /usr/lib64/libQt5Gui.so.5
#28 0x00007ffff6b5626b in QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQt5Gui.so.5
#29 0x00007fffe5d4a52a in xcbSourceDispatch(_GSource*, int (*)(void*), void*) () from /usr/lib64/libQt5XcbQpa.so.5
#30 0x00007ffff51977c7 in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
#31 0x00007ffff5197a50 in g_main_context_iterate.isra () from /usr/lib64/libglib-2.0.so.0
#32 0x00007ffff5197adf in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#33 0x00007ffff674276d in QEventDispatcherGlib::processEvents (this=0x5555556696f0, flags=...) at /var/tmp/portage/dev-qt/qtcore-5.15.2-r2/work/qtbase-everywhere-src-5.15.2/src/corelib/kernel/qeventdispatcher_glib.cpp:423
#34 0x00007ffff66ea493 in QEventLoop::exec (this=this@entry=0x7fffffffdce0, flags=..., flags@entry=...) at /var/tmp/portage/dev-qt/qtcore-5.15.2-r2/work/qtbase-everywhere-src-5.15.2/include/QtCore/../../src/corelib/global/qflags.h:69
#35 0x00007ffff66f23f0 in QCoreApplication::exec () at /var/tmp/portage/dev-qt/qtcore-5.15.2-r2/work/qtbase-everywhere-src-5.15.2/include/QtCore/../../src/corelib/global/qflags.h:121
#36 0x000055555556a9bf in main (argc=<optimized out>, argv=<optimized out>) at /var/tmp/portage/kde-apps/marble-9999/work/marble-9999/src/apps/marble-qt/qtmain.cpp:307


STEPS TO REPRODUCE
1. Choose "Vector OSM" "theme"
2. Zoom out
3. File -> Download Region
4. Click "^" on the second Tile Level Range inputbox (both are 0)
5. Segfault

SOFTWARE/OS VERSIONS
GNU/Linux; Qt version 5.15.2

ADDITIONAL INFORMATION
marble master branch