This issue can be reproduced by following these steps:

1. Open up Assistant.app and go to Preferences
2. Select the Filters tab and click the Add button
3. You will be prompted with a modal dialog where the Cancel and OK buttons are enabled. You should also see the OK button from the parent Preferences dialog. Click on the OK button of the Preferences dialog. Nothing will happen since the current "Add Filter Name" dialog is blocking.
4. Now click on the Cancel button in the "Add Filter Name" dialog. Crash happens here.

Sometimes you may need to click on the OK button of the parent dialog a couple of times before clicking on the Cancel button of the "Add Filter Name" dialog, but the crash is reproducible every time. I am unable to reproduce this issue on OS X 10.8 Mountain Lion.

Below is the stack trace.

Process: Assistant [60837]
Path: /Developer/Applications/Qt/Assistant.app/Contents/MacOS/Assistant
Identifier: com.trolltech.assistant
Version: ???
Code Type: X86-64 (Native)
Parent Process: launchd [173]
Responsible: Assistant [60837]
User ID: 501

Date/Time: 2013-08-12 22:35:23.518 -0400
OS Version: Mac OS X 10.9 (13A538g)
Report Version: 11
Anonymous UUID: BB348B6E-9E86-3E49-4259-A5860B75D265

Crashed Thread: 0 Dispatch queue: com.apple.main-thread

Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: EXC_I386_GPFLT

Application Specific Information:
objc_msgSend() selector name: windowNumber

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 libobjc.A.dylib 0x00007fff8bade097 objc_msgSend + 23
1 com.apple.AppKit 0x00007fff86a94ba6 -[NSApplication _modalSession:sendEvent:] + 311
2 com.apple.AppKit 0x00007fff86a94999 -[NSApplication _realDoModalLoop:peek:] + 726
3 com.apple.AppKit 0x00007fff86acad2c -[NSApplication runModalSession:] + 49
4 QtGui 0x00000001005d84ee QEventDispatcherMac::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 1422
5 QtCore 0x000000010035daf4 QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 68
6 QtCore 0x000000010035dea4 QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) + 324
7 QtGui 0x0000000100ac8875 QDialog::exec() + 229
8 com.trolltech.assistant 0x0000000100035b9c QList<QPair<QString, QStringList> >::append(QPair<QString, QStringList> const&) + 10556
9 QtCore 0x000000010037709e QMetaObject::activate(QObject*, QMetaObject const*, int, void**) + 1566
10 QtGui 0x0000000100ca0ef1 QAbstractButton::clicked(bool) + 49
11 QtGui 0x00000001009c3d50 QAbstractButtonPrivate::emitClicked() + 64
12 QtGui 0x00000001009c4f8a QAbstractButtonPrivate::click() + 154
13 QtGui 0x00000001009c51e1 QAbstractButton::mouseReleaseEvent(QMouseEvent*) + 113
14 QtGui 0x000000010067a467 QWidget::event(QEvent*) + 2423
15 QtGui 0x00000001009c3fd8 QAbstractButton::event(QEvent*) + 40
16 QtGui 0x0000000100a600a5 QPushButton::event(QEvent*) + 69
17 QtGui 0x000000010061f5ed QApplicationPrivate::notify_helper(QObject*, QEvent*) + 189
18 QtGui 0x0000000100626190 QApplication::notify(QObject*, QEvent*) + 2384
19 QtCore 0x000000010035ebdc QCoreApplication::notifyInternal(QObject*, QEvent*) + 124
20 QtGui 0x0000000100623bdd QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool) + 333
21 QtGui 0x00000001005d36cb qt_mac_handleMouseEvent(NSEvent*, QEvent::Type, Qt::MouseButton, QWidget*, bool) + 763
22 com.apple.AppKit 0x00007fff86938215 -[NSWindow sendEvent:] + 781
23 QtGui 0x00000001005c02d7 -[QCocoaPanel sendEvent:] + 87
24 com.apple.AppKit 0x00007fff868d9c8c -[NSApplication sendEvent:] + 2021
25 QtGui 0x00000001005cd814 -[QNSApplication sendEvent:] + 84
26 com.apple.AppKit 0x00007fff86a94999 -[NSApplication _realDoModalLoop:peek:] + 726
27 com.apple.AppKit 0x00007fff86acad2c -[NSApplication runModalSession:] + 49
28 QtGui 0x00000001005d84ee QEventDispatcherMac::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 1422
29 QtCore 0x000000010035daf4 QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 68
30 QtCore 0x000000010035dea4 QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) + 324
31 QtGui 0x0000000100ac8875 QDialog::exec() + 229
32 com.trolltech.assistant 0x0000000100033421 QList<QPair<QString, QStringList> >::append(QPair<QString, QStringList> const&) + 449
33 com.trolltech.assistant 0x000000010002a911 Ui_InstallDialog::setupUi(QDialog*) + 17249
34 QtCore 0x000000010037709e QMetaObject::activate(QObject*, QMetaObject const*, int, void**) + 1566
35 QtGui 0x0000000100619031 QAction::triggered(bool) + 49
36 QtGui 0x000000010061a3c4 QAction::activate(QAction::ActionEvent) + 180
37 QtGui 0x00000001005ccc9a -[QCocoaMenuLoader qtDispatcherToQAction:] + 106
38 com.apple.AppKit 0x00007fff86953cda -[NSApplication sendAction:to:from:] + 327
39 com.apple.AppKit 0x00007fff8696ed68 -[NSMenuItem _corePerformAction] + 394
40 com.apple.AppKit 0x00007fff8696eaa4 -[NSCarbonMenuImpl performActionWithHighlightingForItemAtIndex:] + 117
41 com.apple.AppKit 0x00007fff869bddad -[NSMenu _internalPerformActionForItemAtIndex:] + 35
42 com.apple.AppKit 0x00007fff869bdc21 -[NSCarbonMenuImpl _carbonCommandProcessEvent:handlerCallRef:] + 104
43 com.apple.AppKit 0x00007fff86964c9e NSSLMMenuEventHandler + 716
44 com.apple.HIToolbox 0x00007fff8e249e94 DispatchEventToHandlers(EventTargetRec*, OpaqueEventRef*, HandlerCallRec*) + 892
45 com.apple.HIToolbox 0x00007fff8e249447 SendEventToEventTargetInternal(OpaqueEventRef*, OpaqueEventTargetRef*, HandlerCallRec*) + 385
46 com.apple.HIToolbox 0x00007fff8e25d9b0 SendEventToEventTarget + 40
47 com.apple.HIToolbox 0x00007fff8e293710 SendHICommandEvent(unsigned int, HICommand const*, unsigned int, unsigned int, unsigned char, void const*, OpaqueEventTargetRef*, OpaqueEventTargetRef*, OpaqueEventRef**) + 420
48 com.apple.HIToolbox 0x00007fff8e2c6278 SendMenuCommandWithContextAndModifiers + 59
49 com.apple.HIToolbox 0x00007fff8e2c6228 SendMenuItemSelectedEvent + 178
50 com.apple.HIToolbox 0x00007fff8e2c6109 FinishMenuSelection(SelectionData*, MenuResult*, MenuResult*) + 94
51 com.apple.HIToolbox 0x00007fff8e2ce0e5 MenuSelectCore(MenuData*, Point, double, unsigned int, OpaqueMenuRef*, unsigned short) + 718
52 com.apple.HIToolbox 0x00007fff8e2cdd11 _HandleMenuSelection2 + 446
53 com.apple.AppKit 0x00007fff868d7cbc _NSHandleCarbonMenuEvent + 284
54 com.apple.AppKit 0x00007fff867378de _DPSNextEvent + 2170
55 com.apple.AppKit 0x00007fff86736c4b -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 122
56 com.apple.AppKit 0x00007fff8672abbc -[NSApplication run] + 553
57 QtGui 0x00000001005d8680 QEventDispatcherMac::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 1824
58 QtCore 0x000000010035daf4 QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 68
59 QtCore 0x000000010035dea4 QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) + 324
60 QtCore 0x000000010036058c QCoreApplication::exec() + 188
61 com.trolltech.assistant 0x000000010002a5a3 Ui_InstallDialog::setupUi(QDialog*) + 16371
62 com.trolltech.assistant 0x0000000100003b68 0x100000000 + 15208