Confirming with the following backtrace:

Application: Umbrello UML Modeller (umbrello), signal SIGSEGV
[Thread debugging using libthread_db enabled]
[New Thread 0xb5ef7700 (LWP 9310)]
[KCrash handler]
#6  QString (this=0xbfeedb7c, other=@0x18)
    at /usr/include/qt4/QtCore/qstring.h:676
#7  0x0832f136 in UMLObject::getName (this=0x0)
    at /var/tmp/portage/kde-base/umbrello-4.1.0/work/umbrello-4.1.0/umbrello/umbrello/umlobject.cpp:125
#8  0x082837db in UMLClassifier::findOperation (this=0x865ed68, 
    name=@0xbfeedc20, params=
          {<QLinkedList<Model_Utils::NameAndType>> = {{d = 0xbfeedc30, e = 0xbfeedc30}}, <No data fields>})
    at /var/tmp/portage/kde-base/umbrello-4.1.0/work/umbrello-4.1.0/umbrello/umbrello/classifier.cpp:157
#9  0x082ae2f0 in FloatingTextWidget::showOpDlg (this=0x86e1158)
    at /var/tmp/portage/kde-base/umbrello-4.1.0/work/umbrello-4.1.0/umbrello/umbrello/floatingtextwidget.cpp:327
#10 0x082fc77a in ToolBarStateMessages::setSecondWidget (this=0x86e18c8, 
    secondObject=0x864fcc0, messageType=ToolBarStateMessages::NormalMessage)
    at /var/tmp/portage/kde-base/umbrello-4.1.0/work/umbrello-4.1.0/umbrello/umbrello/toolbarstatemessages.cpp:215
#11 0x082fce4a in ToolBarStateMessages::mouseReleaseWidget (this=0x86e18c8)
    at /var/tmp/portage/kde-base/umbrello-4.1.0/work/umbrello-4.1.0/umbrello/umbrello/toolbarstatemessages.cpp:111
#12 0x082fa71d in ToolBarState::mouseRelease (this=0x86e18c8, ome=0xbfeedd48)
    at /var/tmp/portage/kde-base/umbrello-4.1.0/work/umbrello-4.1.0/umbrello/umbrello/toolbarstate.cpp:86
#13 0xb7d8ddc2 in Q3ScrollView::viewportMouseReleaseEvent (this=0x86fbd08, 
    e=0xbfeee330) at widgets/q3scrollview.cpp:1761
#14 0xb7d8efed in Q3ScrollView::eventFilter (this=0x86fbd08, obj=0x86dd110, 
    e=0xbfeee330) at widgets/q3scrollview.cpp:1479
#15 0xb72e4192 in QCoreApplicationPrivate::sendThroughObjectEventFilters (
    this=0x846cbe8, receiver=0x86dd110, event=0xbfeee330)
    at kernel/qcoreapplication.cpp:694
#16 0xb6450884 in QApplicationPrivate::notify_helper (this=0x846cbe8, 
    receiver=0x86dd110, e=0xbfeee330) at kernel/qapplication.cpp:3768
#17 0xb6454b60 in QApplication::notify (this=0xbfeee90c, receiver=0x86dd110, 
    e=0xbfeee330) at kernel/qapplication.cpp:3501
#18 0xb78bb6c3 in KApplication::notify (this=0xbfeee90c, receiver=0x86dd110, 
    event=0xbfeee330)
    at /var/tmp/portage/kde-base/kdelibs-4.1.0/work/kdelibs-4.1.0/kdeui/kernel/kapplication.cpp:311
#19 0xb72e3d59 in QCoreApplication::notifyInternal (this=0xbfeee90c, 
    receiver=0x86dd110, event=0xbfeee330) at kernel/qcoreapplication.cpp:587
#20 0xb645667f in QApplicationPrivate::sendMouseEvent (receiver=0x86dd110, 
    event=0xbfeee330, alienWidget=0x86dd110, nativeWidget=0x850d2e0, 
    buttonDown=0xb6c37c40, lastMouseReceiver=@0xb6c37c44)
    at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:218
#21 0xb64bb91e in QETWidget::translateMouseEvent (this=0x850d2e0, 
    event=0xbfeee728) at kernel/qapplication_x11.cpp:4133
#22 0xb64ba48d in QApplication::x11ProcessEvent (this=0xbfeee90c, 
    event=0xbfeee728) at kernel/qapplication_x11.cpp:3133
#23 0xb64e0ed8 in QEventDispatcherX11::processEvents (this=0x846ccf0, 
    flags=@0xbfeee818) at kernel/qeventdispatcher_x11.cpp:134
#24 0xb72e30d3 in QEventLoop::processEvents (this=0xbfeee890, 
    flags=@0xbfeee858) at kernel/qeventloop.cpp:149
#25 0xb72e3246 in QEventLoop::exec (this=0xbfeee890, flags=@0xbfeee898)
    at kernel/qeventloop.cpp:200
#26 0xb72e5401 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:845
#27 0xb645023f in QApplication::exec () at kernel/qapplication.cpp:3304
#28 0x082cf245 in main (argc=1, argv=0xbfeeea64)
    at /var/tmp/portage/kde-base/umbrello-4.1.0/work/umbrello-4.1.0/umbrello/umbrello/main.cpp:110
#0  0xffffe424 in __kernel_vsyscall ()

Looks like a duplicate of bug 131373

Created attachment 59179 [details]
New crash information added by DrKonqi

umbrello (2.5.5) on KDE Platform 4.5.5 (KDE 4.5.5) using Qt 4.6.3

- What I was doing when the application crashed:
add syncronus messagge with parameter without data type.

-- Backtrace (Reduced):
#6  QString (this=0x0) at /usr/include/QtCore/qstring.h:714
#7  UMLObject::getName (this=0x0) at /usr/src/debug/kdesdk-4.5.5/umbrello/umbrello/umlobject.cpp:186
#8  0x0000000000a77bd5 in UMLClassifier::findOperation (this=<value optimized out>, name=<value optimized out>, params=...) at /usr/src/debug/kdesdk-4.5.5/umbrello/umbrello/classifier.cpp:193
#9  0x0000000000a45f9d in FloatingTextWidget::showOperationDialog (this=0x2e1b700) at /usr/src/debug/kdesdk-4.5.5/umbrello/umbrello/widgets/floatingtextwidget.cpp:358
#10 0x0000000000ac7e2b in ToolBarStateMessages::setSecondWidget (this=0x2e85230, secondObject=0x32d77e0, messageType=<value optimized out>)
    at /usr/src/debug/kdesdk-4.5.5/umbrello/umbrello/toolbarstatemessages.cpp:293

At the second stack frame in /usr/src/debug/kdesdk-4.5.5/umbrello/umbrello/classifier.cpp around 193

            Model_Utils::NameAndType_ListIt nt(params.begin() + i);
            UMLClassifier *c = dynamic_cast<UMLClassifier*>((*nt).m_type);
>            UMLClassifier *testType = testParams.at(i)->getType();
            if (c == NULL) {  //template parameter
                if (testType->name() != "class")
                    break;

in the related case the testType param is zero and let crash theaccess to name() in the following line

                if (testType->name() != "class")

fixed in svn

*** Bug 272276 has been marked as a duplicate of this bug. ***

apply fixed bug from 4.9.0 changelog