designer crashes when loading due to problems in phonon

When loading designer, I get a segmentation fault in libphonon. The line where the segfault occurs is 3rdparty/phonon/phonon/videowidget.cpp:158. This line contains
pINTERFACE_CALL(setAspectRatio(aspectRatio));

pINTERFACE_CALL expands to
#define pINTERFACE_CALL(function) \
Iface<PHONON_INTERFACENAME >::cast(this)->function

For me, the call to Iface<PHONON_INTERFACENAME >::cast(this) returns a null pointer. Then the subsequent dereference of that pointer segfaults.

Program received signal SIGSEGV, Segmentation fault.
0xb332574d in Phonon::VideoWidgetPrivate::setupBackendObject (this=0x98ceb60) at ../3rdparty/phonon/phonon/videowidget.cpp:158
158 pINTERFACE_CALL(setAspectRatio(aspectRatio));
(gdb) bt full
#0 0xb332574d in Phonon::VideoWidgetPrivate::setupBackendObject (this=0x98ceb60) at ../3rdparty/phonon/phonon/videowidget.cpp:158
q = (Phonon::VideoWidget * const) 0x9877288
w = (class QWidget *) 0x80d63e4
#1 0xb33261c6 in Phonon::VideoWidgetPrivate::createBackendObject (this=0x98ceb60) at ../3rdparty/phonon/phonon/videowidget.cpp:77
q = (Phonon::VideoWidget * const) 0x9877288
#2 0xb3326485 in VideoWidget (this=0x9877288, parent=0x9855d10) at ../3rdparty/phonon/phonon/videowidget.cpp:47
No locals.
#3 0xb33275ee in Phonon::VideoPlayerPrivate::init (this=0x987e890, q=0x9855d10, category=Phonon::NoCategory)
at ../3rdparty/phonon/phonon/videoplayer.cpp:61
layout = (class QVBoxLayout *) 0x98658a0
#4 0xb3327a6d in VideoPlayer (this=0x9855d10, category=Phonon::NoCategory, parent=0x0) at ../3rdparty/phonon/phonon/videoplayer.cpp:75
No locals.
#5 0xb421eb4b in VideoPlayerPlugin::createWidget () from /opt/qt-4.6.0/plugins/designer/libphononwidgets.so
No symbol table info available.
#6 0xb70c0dc0 in qdesigner_internal::WidgetFactory::createCustomWidget (this=0x92be900, className=@0xbfe2e0cc, parentWidget=0x0, creationError=0xbfe2dfc7)
at shared/widgetfactory.cpp:259
it =

{i = 0x92e1a58}

factory = (QDesignerCustomWidgetInterface *) 0x92d1600
rc = (class QWidget *) 0x1ab09a4
lang = (QDesignerLanguageExtension *) 0xbfe2de28
createdClassNameC = 0xb6e5e08e "\201��zs"
classNameB = {static shared_null = {ref = {_q_value = 1970}, alloc = 0, size = 0, data = 0x80d6328 "", array = ""}, static shared_empty = {ref =

{ _q_value = 20}

, alloc = 0, size = 0, data = 0xb5cd3060 "", array = ""}, d = 0x92a28cc}
classNameC = 0xb588f140 ""
knownCustomClasses = {q_hash = d = 0x984f1c8, e = 0x984f1c8}
#7 0xb70c16a9 in qdesigner_internal::WidgetFactory::createWidget (this=0x92be900, widgetName=@0xbfe2e0cc, parentWidget=0x0)
at shared/widgetfactory.cpp:329
widgetNameBA = {static shared_null = {ref = {_q_value = 1970}, alloc = 0, size = 0, data = 0x80d6328 "", array = ""}, static shared_empty = {
ref = {_q_value = 20}, alloc = 0, size = 0, data = 0xb5cd3060 "", array = ""}, d = 0xbfe2e000}
fallBackBaseClass = {static null =

{<No data fields>}, static shared_null = {ref = {_q_value = 9028}, alloc = 0, size = 0, data = 0x80d6242,
clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, static shared_empty = {ref = {_q_value = 7649},
alloc = 0, size = 0, data = 0xb5cd314e, clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}},
d = 0x92af7f8, static codecForCStrings = 0x0}
db = (class QDesignerWidgetDataBaseInterface *) 0x0
baseClass = {static null = {<No data fields>}

, static shared_null = {ref = {_q_value = 9028}, alloc = 0, size = 0, data = 0x80d6242, clean = 0,
simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, static shared_empty = {ref = {_q_value = 7649}, alloc = 0,
size = 0, data = 0xb5cd314e, clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, d = 0xb501fd68,
static codecForCStrings = 0x0}
customWidgetCreationError = false
widgetNameC = 0xb705ff1b "\203�\024[]�U\211�S\203�\024�zs��\201�C
S"
item = (QDesignerWidgetDataBaseItemInterface *) 0x92be900
fw = (class QDesignerFormWindowInterface *) 0x0
w = (class QWidget *) 0x0
_PRETTY_FUNCTION_ = "virtual QWidget* qdesigner_internal::WidgetFactory::createWidget(const QString&, QWidget*) const"
#8 0xb70ca4ea in qdesigner_internal::WidgetDataBase::defaultPropertyValues (this=0x92ad660, name=@0xbfe2e0cc) at shared/widgetdatabase.cpp:447
factory = (class qdesigner_internal::WidgetFactory *) 0x92be900
object = (class QObject *) 0x0
result = p = {static shared_null = {ref = {_q_value = 5339}, alloc = 0, begin = 0, end = 0, sharable = 1, array = {0x0, d = 0x47}, d = 0x47}}
#9 0xb70ca73e in qdesigner_internal::WidgetDataBase::grabDefaultPropertyValues (this=0x92ad660) at shared/widgetdatabase.cpp:469
dbItem = (QDesignerWidgetDataBaseItemInterface *) 0x92d59d0
default_prop_values = p = {static shared_null = {ref = {_q_value = 5339}, alloc = 0, begin = 0, end = 0, sharable = 1, array = {0x0,
d = 0x9883050}, d = 0x9883050}}
i = 71
itemCount = 152
#10 0xb708eeb3 in qdesigner_internal::QDesignerIntegration::initializePlugins (formEditor=0x92391a8) at shared/qdesigner_integration.cpp:366
widgetDataBase = (class qdesigner_internal::WidgetDataBase *) 0x92ad660
#11 0xb75e6ca1 in QDesignerComponents::initializePlugins (core=0x92391a8) at qdesigner_components.cpp:142
No locals.
#12 0x080829e4 in QDesignerWorkbench (this=0x92ad870) at qdesigner_workbench.cpp:192
No locals.
#13 0x08076d5b in QDesigner::initialize (this=0xbfe2e404) at qdesigner.cpp:243
files = {<QList<QString>> = p = {static shared_null = {ref = {_q_value = 5339}, alloc = 0, begin = 0, end = 0, sharable = 1, array = {0x0,
d = 0x80d63e4}, d = 0x80d63e4}}, <No data fields>}
resourceDir = {static null =

{<No data fields>}, static shared_null = {ref = {_q_value = 9028}, alloc = 0, size = 0, data = 0x80d6242, clean = 0,
simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, static shared_empty = {ref = {_q_value = 7649}, alloc = 0,
size = 0, data = 0xb5cd314e, clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, d = 0x9263900,
static codecForCStrings = 0x0}
translator = (class QTranslator *) 0x92a2f08
qtTranslator = (class QTranslator *) 0x92abf60
localSysName = {static null = {<No data fields>}

, static shared_null = {ref = {_q_value = 9028}, alloc = 0, size = 0, data = 0x80d6242, clean = 0,
simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, static shared_empty = {ref = {_q_value = 7649}, alloc = 0,
size = 0, data = 0xb5cd314e, clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, d = 0x91fe9c0,
static codecForCStrings = 0x0}
translatorFileName = {static null =

{<No data fields>}, static shared_null = {ref = {_q_value = 9028}, alloc = 0, size = 0, data = 0x80d6242,
clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, static shared_empty = {ref = {_q_value = 7649},
alloc = 0, size = 0, data = 0xb5cd314e, clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}},
d = 0x9263a50, static codecForCStrings = 0x0}
#14 0x08077b68 in QDesigner (this=0xbfe2e404, argc=@0xbfe2e450, argv=0xbfe2e4d4) at qdesigner.cpp:99
No locals.
#15 0x08075b2c in main (argc=1, argv=0xbfe2e4d4) at main.cpp:54
app = {<QApplication> = {<QCoreApplication> = {<QObject> = {_vptr.QObject = 0x80bcec8, static staticMetaObject = {d = {superdata = 0x0,
stringdata = 0xb5c55a60 "QObject", data = 0xb5c55b00, extradata = 0xb5ccb3c0}}, d_ptr = {d = 0x91fae78}, static staticQtMetaObject = {d = { superdata = 0x0, stringdata = 0xb5c60760 "Qt", data = 0xb5c63d20, extradata = 0x0}}}, static staticMetaObject = {d = {superdata = 0x80d6160,
stringdata = 0xb5c69be0 "QCoreApplication", data = 0xb5c69c80, extradata = 0x0}}, static self = 0xbfe2e404}, static staticMetaObject = {d = { superdata = 0xb5cd0564, stringdata = 0xb683b840 "QApplication", data = 0xb683ba80, extradata = 0x0}}}, static staticMetaObject = {d = {
superdata = 0x80d6140, stringdata = 0x80bcdc0 "QDesigner", data = 0x80bce20, extradata = 0x0}}, m_server = 0x0, m_client = 0x0, m_workbench = 0x0,
m_mainWindow = {o = 0x0}, m_errorMessageDialog = {o = 0x0}, m_initializationErrors = {static null = {<No data fields>}

, static shared_null = {ref =

{ _q_value = 9028}

, alloc = 0, size = 0, data = 0x80d6242, clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0,
array = {0}}, static shared_empty = {ref = {_q_value = 7649}, alloc = 0, size = 0, data = 0xb5cd314e, clean = 0, simpletext = 0, righttoleft = 0,
asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, d = 0x80d6230, static codecForCStrings = 0x0}, m_lastErrorMessage = {
static null =

{<No data fields>}

, static shared_null = {ref = {_q_value = 9028}, alloc = 0, size = 0, data = 0x80d6242, clean = 0, simpletext = 0,
righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, static shared_empty = {ref = {_q_value = 7649}, alloc = 0, size = 0,
data = 0xb5cd314e, clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, d = 0x80d6230,
static codecForCStrings = 0x0}, m_suppressNewFormShow = false}