Version:           4.3.0 (using KDE 4.3.0)
OS:                Linux
Installed from:    Ubuntu Packages

Hi ! This seems to be an easily reproductible issue.

Assume you have a directory in a folderview widget. When the mouse hovers over the folder icon, a popup window displays the content of the directory (new feature in 4.3.0).

If you delete / move this directory in the folderview widget, but immediatly move the mouse over it (so as to see the preview of its content) a popup window with a red button appears : "This file or folder doesn't exist".

Then plasma crashes and restarts. Here's a debug log :

Application: Plasma Workspace (kdeinit4), signal: Segmentation fault
[Current thread is 0 (LWP 3403)]

Thread 2 (Thread 0xa7d42b90 (LWP 3406)):
#0  0xb7fba430 in __kernel_vsyscall ()
#1  0xb64da0e5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib/tls/i686/cmov/libpthread.so.0
#2  0xb66ba2ed in pthread_cond_wait () from /lib/tls/i686/cmov/libc.so.6
#3  0xb7dbc172 in QWaitCondition::wait (this=0x936fd48, mutex=0x936fd44, time=4294967295) at thread/qwaitcondition_unix.cpp:87
#4  0xb76fcac2 in QHostInfoAgent::run (this=0x936fd38) at kernel/qhostinfo.cpp:260
#5  0xb7dbb132 in QThreadPrivate::start (arg=0x936fd38) at thread/qthread_unix.cpp:188
#6  0xb64d64ff in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
#7  0xb66ab49e in clone () from /lib/tls/i686/cmov/libc.so.6

Thread 1 (Thread 0xb5fe4a10 (LWP 3403)):
[KCrash Handler]
#6  QGraphicsScene::drawItems (this=0xa549e70, painter=0xbfad6320, numItems=2, items=0x97f5130, options=0xa61fc40, widget=0x8c563c8) at graphicsview/qgraphicsscene.cpp:5020
#7  0xb6fb861c in QGraphicsView::drawItems (this=0x968cf90, painter=0xbfad6320, numItems=2, items=0x97f5130, options=0xa61fc40) at graphicsview/qgraphicsview.cpp:3777
#8  0xb6fc5022 in QGraphicsView::paintEvent (this=0x968cf90, event=0xbfad68a4) at graphicsview/qgraphicsview.cpp:3528
#9  0xb69c774e in QWidget::event (this=0x968cf90, event=0xbfad68a4) at kernel/qwidget.cpp:7687
#10 0xb6d78a53 in QFrame::event (this=0x968cf90, e=0xbfad68a4) at widgets/qframe.cpp:559
#11 0xb6e18c2f in QAbstractScrollArea::viewportEvent (this=0x968cf90, e=0x4) at widgets/qabstractscrollarea.cpp:962
#12 0xb6fc39b2 in QGraphicsView::viewportEvent (this=0x968cf90, event=0xbfad68a4) at graphicsview/qgraphicsview.cpp:2919
#13 0xb6e1b235 in QAbstractScrollAreaFilter::eventFilter (this=0xa1ab020, o=0x8c563c8, e=0xbfad68a4) at widgets/qabstractscrollarea_p.h:100
#14 0xb7eaedea in QCoreApplicationPrivate::sendThroughObjectEventFilters (this=0x8baa350, receiver=0x8c563c8, event=0xbfad68a4) at kernel/qcoreapplication.cpp:726
#15 0xb6970d1a in QApplicationPrivate::notify_helper (this=0x8baa350, receiver=0x8c563c8, e=0xbfad68a4) at kernel/qapplication.cpp:4052
#16 0xb6979122 in QApplication::notify (this=0x8ba46b8, receiver=0x8c563c8, e=0xbfad68a4) at kernel/qapplication.cpp:4021
#17 0xb74815cd in KApplication::notify (this=0x8ba46b8, receiver=0x8c563c8, event=0xbfad68a4) at /build/buildd/kde4libs-4.3.0/kdeui/kernel/kapplication.cpp:302
#18 0xb7eafbcb in QCoreApplication::notifyInternal (this=0x8ba46b8, receiver=0x8c563c8, event=0xbfad68a4) at kernel/qcoreapplication.cpp:610
#19 0xb697be8e in QCoreApplication::sendSpontaneousEvent (receiver=0x8c563c8, event=0x4) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:216
#20 0xb69cf4e9 in QWidgetPrivate::drawWidget (this=0x97dbd20, pdev=0x97850fc, rgn=@0xbfad6a04, offset=@0xbfad69fc, flags=4, sharedPainter=0x0, backingStore=0xa50cec0) at kernel/qwidget.cpp:5079
#21 0xb69cfc8e in QWidgetPrivate::paintSiblingsRecursive (this=0xa60ddc0, pdev=0x97850fc, siblings=@0xa60ddcc, index=2, rgn=@0xbfad6be4, offset=@0xbfad6bdc, flags=4, sharedPainter=0x0, 
    backingStore=0xa50cec0) at kernel/qwidget.cpp:5189
#22 0xb69cf0da in QWidgetPrivate::drawWidget (this=0xa60ddc0, pdev=0x97850fc, rgn=@0xbfad6be4, offset=@0xbfad6bdc, flags=4, sharedPainter=0x0, backingStore=0xa50cec0) at kernel/qwidget.cpp:5128
#23 0xb69cfc8e in QWidgetPrivate::paintSiblingsRecursive (this=0xa5e2000, pdev=0x97850fc, siblings=@0xa5e200c, index=3, rgn=@0xbfad6e98, offset=@0xa50cee4, flags=4, sharedPainter=0x0, 
    backingStore=0xa50cec0) at kernel/qwidget.cpp:5189
#24 0xb69cf0da in QWidgetPrivate::drawWidget (this=0xa5e2000, pdev=0x97850fc, rgn=@0xbfad6e98, offset=@0xa50cee4, flags=4, sharedPainter=0x0, backingStore=0xa50cec0) at kernel/qwidget.cpp:5128
#25 0xb6b609ca in QWidgetBackingStore::sync (this=0xa50cec0) at painting/qbackingstore.cpp:1269
#26 0xb69bfa06 in QWidgetPrivate::syncBackingStore (this=0xa5e2000) at kernel/qwidget.cpp:1603
#27 0xb69c7bb5 in QWidget::event (this=0xa5c1628, event=0xa486cc8) at kernel/qwidget.cpp:7827
#28 0xb6970d3c in QApplicationPrivate::notify_helper (this=0x8baa350, receiver=0xa5c1628, e=0xa486cc8) at kernel/qapplication.cpp:4056
#29 0xb6979122 in QApplication::notify (this=0x8ba46b8, receiver=0xa5c1628, e=0xa486cc8) at kernel/qapplication.cpp:4021
#30 0xb74815cd in KApplication::notify (this=0x8ba46b8, receiver=0xa5c1628, event=0xa486cc8) at /build/buildd/kde4libs-4.3.0/kdeui/kernel/kapplication.cpp:302
#31 0xb7eafbcb in QCoreApplication::notifyInternal (this=0x8ba46b8, receiver=0xa5c1628, event=0xa486cc8) at kernel/qcoreapplication.cpp:610
#32 0xb7eb0825 in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x8b2bb38) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:213
#33 0xb7eb0a1d in QCoreApplication::sendPostedEvents (receiver=0x0, event_type=0) at kernel/qcoreapplication.cpp:1140
#34 0xb7edb6af in postEventSourceDispatch (s=0x8bac510) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:218
#35 0xb6523b88 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#36 0xb65270eb in ?? () from /usr/lib/libglib-2.0.so.0
#37 0xb6527268 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#38 0xb7edb2f8 in QEventDispatcherGlib::processEvents (this=0x8baa068, flags={i = -1079151224}) at kernel/qeventdispatcher_glib.cpp:327
#39 0xb6a12a75 in QGuiEventDispatcherGlib::processEvents (this=0x8baa068, flags={i = -1079151176}) at kernel/qguieventdispatcher_glib.cpp:202
#40 0xb7eae1fa in QEventLoop::processEvents (this=0xbfad7630, flags={i = -1079151112}) at kernel/qeventloop.cpp:149
#41 0xb7eae642 in QEventLoop::exec (this=0xbfad7630, flags={i = -1079151048}) at kernel/qeventloop.cpp:201
#42 0xb7eb0ae9 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:888
#43 0xb6970bb7 in QApplication::exec () at kernel/qapplication.cpp:3525
#44 0xb3b9c120 in kdemain () from /usr/lib/libkdeinit4_plasma-desktop.so
#45 0x0804e27d in launch (argc=1, _name=0x8b2c57c "/usr/bin/plasma-desktop", args=0x8b2c594 "", cwd=0x0, envc=0, envs=0x8b2c598 "", reset_env=false, tty=0x0, avoid_loops=false, 
    startup_id_str=0x805136d "0") at /build/buildd/kde4libs-4.3.0/kinit/kinit.cpp:676
#46 0x0804ea5d in handle_launcher_request (sock=8, who=<value optimized out>) at /build/buildd/kde4libs-4.3.0/kinit/kinit.cpp:1168
#47 0x0804efe5 in handle_requests (waitForPid=0) at /build/buildd/kde4libs-4.3.0/kinit/kinit.cpp:1361
#48 0x0804fbca in main (argc=2, argv=0xbfad7e24, envp=0xbfad7e30) at /build/buildd/kde4libs-4.3.0/kinit/kinit.cpp:1788

Thanks for reading this & for your amazing work :)