Cannot confirm.
Tried with an existing XMI file, and also tried with two new classes
created ad-hoc.

Perhaps you could attach the XMI file for which this happens to you.

Ouch. Forget my previous comments, I do confirm.

#0  0x0816bbe1 in FloatingTextWidget::showOpDlg (this=0x0)
    at /kdesdk-3.5-branch/umbrello/umbrello/floatingtextwidget.cpp:272
#1  0x081acc7b in ToolBarStateMessages::setSecondWidget (this=0x8744ea0, 
    secondObject=0x89def50, messageType=NormalMessage)
    at /kdesdk-3.5-branch/umbrello/umbrello/toolbarstatemessages.cpp:147
#2  0x081aca01 in ToolBarStateMessages::mouseReleaseWidget (this=0x8744ea0)
    at /kdesdk-3.5-branch/umbrello/umbrello/toolbarstatemessages.cpp:106
#3  0x081a9979 in ToolBarState::mouseRelease (this=0x8744ea0, ome=0xbfffe210)
    at /kdesdk-3.5-branch/umbrello/umbrello/toolbarstate.cpp:84
#4  0x081e2de2 in UMLView::contentsMouseReleaseEvent (this=0x89ae888, 
    ome=0xbfffe210)
    at /kdesdk-3.5-branch/umbrello/umbrello/umlview.cpp:381
#5  0x40f81168 in QScrollView::viewportMouseReleaseEvent ()
    from /usr/lib/qt3/lib/libqt-mt.so.3

SVN commit 654857 by okellogg:

activate(): Attempt resolving m_widget{A,B}Id only if the m_pOw[] is NULL.
BUG:144293


 M  +1 -0      ChangeLog  
 M  +26 -22    umbrello/messagewidget.cpp  


--- branches/KDE/3.5/kdesdk/umbrello/ChangeLog #654856:654857
@@ -30,6 +30,7 @@
 * Support duplication of diagrams (139856, 143581)
 * Crash on changing multiplicity in an association in ERD (143909)
 * Class diagram in folder not loaded correctly from xmi (144119)
+* Sequence diagram crashs during message inserting (144293)
 
 Version 1.5.61
 
--- branches/KDE/3.5/kdesdk/umbrello/umbrello/messagewidget.cpp #654856:654857
@@ -407,35 +407,39 @@
 bool MessageWidget::activate(IDChangeLog * Log /*= 0*/) {
     m_pView->resetPastePoint();
     // UMLWidget::activate(Log);   CHECK: I don't think we need this ?
-    UMLWidget *pWA = m_pView->findWidget(m_widgetAId);
-    if (pWA == NULL) {
-        kDebug() << "MessageWidget::activate: role A object "
-            << ID2STR(m_widgetAId) << " not found" << endl;
-        return false;
-    }
-    UMLWidget *pWB = m_pView->findWidget(m_widgetBId);
-    if (pWB == NULL) {
-        kDebug() << "MessageWidget::activate: role B object "
-            << ID2STR(m_widgetBId) << " not found" << endl;
-        return false;
-    }
-    m_pOw[Uml::A] = dynamic_cast<ObjectWidget*>(pWA);
     if (m_pOw[Uml::A] == NULL) {
-        kDebug() << "MessageWidget::activate: role A widget "
-            << ID2STR(m_widgetAId) << " is not an ObjectWidget" << endl;
-        return false;
+        UMLWidget *pWA = m_pView->findWidget(m_widgetAId);
+        if (pWA == NULL) {
+            kDebug() << "MessageWidget::activate: role A object "
+                << ID2STR(m_widgetAId) << " not found" << endl;
+            return false;
+        }
+        m_pOw[Uml::A] = dynamic_cast<ObjectWidget*>(pWA);
+        if (m_pOw[Uml::A] == NULL) {
+            kDebug() << "MessageWidget::activate: role A widget "
+                << ID2STR(m_widgetAId) << " is not an ObjectWidget" << endl;
+            return false;
+        }
     }
-    m_pOw[Uml::B] = dynamic_cast<ObjectWidget*>(pWB);
     if (m_pOw[Uml::B] == NULL) {
-        kDebug() << "MessageWidget::activate: role B widget "
-            << ID2STR(m_widgetBId) << " is not an ObjectWidget" << endl;
-        return false;
+        UMLWidget *pWB = m_pView->findWidget(m_widgetBId);
+        if (pWB == NULL) {
+            kDebug() << "MessageWidget::activate: role B object "
+                << ID2STR(m_widgetBId) << " not found" << endl;
+            return false;
+        }
+        m_pOw[Uml::B] = dynamic_cast<ObjectWidget*>(pWB);
+        if (m_pOw[Uml::B] == NULL) {
+            kDebug() << "MessageWidget::activate: role B widget "
+                << ID2STR(m_widgetBId) << " is not an ObjectWidget" << endl;
+            return false;
+        }
     }
     updateResizability();
 
-    UMLClassifier *c = dynamic_cast<UMLClassifier*>(pWB->getUMLObject());
+    UMLClassifier *c = dynamic_cast<UMLClassifier*>(m_pOw[Uml::B]->getUMLObject());
     UMLOperation *op = NULL;
-    if (c) {
+    if (c && !m_CustomOp.isEmpty()) {
         Uml::IDType opId = STR2ID(m_CustomOp);
         op = dynamic_cast<UMLOperation*>( c->findChildObjectById(opId, true) );
         if (op) {