crash when creating scroll bar skin graphics on S60

Starting a Qt app with scroll bars (eg collidingmice) on the new MCL gives the following crash:

46 User::Panic() us_func.cpp:741 0x60016b50
45 AknIconUtils::CreateIconLC() AknIconUtils.cpp:1071 0x124a8f53
44 AknIconUtils::CreateIconLC() AknIconUtils.cpp:108 0x124a72e9
43 AknsUtils::CreateIconLC() AknsUtils.cpp:192 0x121d12d8
42 AknsUtils::CreateIconL() AknsUtils.cpp:144 0x121d11dd
41 QS60StyleModeSpecifics::createSkinnedGraphicsLX() qs60style_s60.cpp:826 0x3f7f7b43
40 QS60StyleModeSpecifics::skinnedGraphics() qs60style_s60.cpp:417 0x3f7f5e43
39 QS60StylePrivate::part() qs60style_s60.cpp:1362 0x3f7f9bf5
38 QS60StylePrivate::cachedPart() qs60style.cpp:612 0x3f7e9521
37 QS60StylePrivate::drawPart() qs60style.cpp:534 0x3f7e8dd5
36 QS60StylePrivate::drawRow() qs60style.cpp:595 0x3f7e92a6
35 QS60StylePrivate::drawSkinElement() qs60style.cpp:190 0x3f7e7652
34 QS60Style::drawComplexControl() qs60style.cpp:1003 0x3f7eae56
33 QScrollBar::paintEvent() qscrollbar.cpp:566 0x3f99cc62
32 QWidget::event() qwidget.cpp:8432 0x3f64dedc
31 QAbstractSlider::event() qabstractslider.cpp:942 0x3f8f3725
30 QScrollBar::event() qscrollbar.cpp:546 0x3f99cb09
29 QApplicationPrivate::notify_helper() qapplication.cpp:4474 0x3f5ff00c
28 QApplication::notify() qapplication.cpp:4439 0x3f5fe51f
27 QCoreApplication::notifyInternal() qcoreapplication.cpp:759 0x3a7986db
26 QCoreApplication::sendSpontaneousEvent() qcoreapplication.h:234 0x3a70b116
25 QWidgetPrivate::drawWidget() qwidget.cpp:5514 0x3f64714f
24 QWidgetPrivate::paintSiblingsRecursive() qwidget.cpp:5722 0x3f647efe
23 QWidgetPrivate::drawWidget() qwidget.cpp:5568 0x3f6474e1
22 QWidgetPrivate::paintSiblingsRecursive() qwidget.cpp:5722 0x3f647efe
21 QWidgetPrivate::drawWidget() qwidget.cpp:5568 0x3f6474e1
20 QWidgetBackingStore::sync() qbackingstore.cpp:1371 0x3f76eb5f
19 QWidgetBackingStore::sync() qbackingstore.cpp:1157 0x3f76ddeb
18 QWidgetPrivate::syncBackingStore() qwidget.cpp:1865 0x3f63f033
17 QSymbianControl::Draw() qapplication_s60.cpp:1078 0x3f66d74d
16 CCoeControl::HandleRedrawEvent() COECNTRL.CPP:1411 0x40606191
15 CCoeRedrawer::RunL() COEPRIV.CPP:80 0x4061a57c

The assert in AknIconUtils::CreateIconLC() is because aBitmapId < 0 (== -1).

This number comes from a call to fallbackInfo(part, fallbackGraphicID); where part is SP_QsnCpScrollBgBottom
and fallbackGraphicID ends up as -1.