### Eclipse Workspace Patch 1.0
#P org.eclipse.jface
Index: src/org/eclipse/jface/viewers/AbstractTreeViewer.java
===================================================================
RCS file: /cvsroot/eclipse/org.eclipse.jface/src/org/eclipse/jface/viewers/AbstractTreeViewer.java,v
retrieving revision 1.153
diff -u -r1.153 AbstractTreeViewer.java
--- src/org/eclipse/jface/viewers/AbstractTreeViewer.java	30 Mar 2011 18:23:45 -0000	1.153
+++ src/org/eclipse/jface/viewers/AbstractTreeViewer.java	12 Apr 2011 13:11:40 -0000
@@ -39,6 +39,7 @@
 import org.eclipse.swt.graphics.Point;
 import org.eclipse.swt.widgets.Control;
 import org.eclipse.swt.widgets.Item;
+import org.eclipse.swt.widgets.TreeItem;
 import org.eclipse.swt.widgets.Widget;
 
 /**
@@ -272,6 +273,7 @@
 			}
 			createAddedElements(widget, filtered);
 		}
+		assertElementsNotNull(parent, getChildren(widget));
 	}
 
 	/**
@@ -1390,7 +1392,12 @@
 				&& elements.length > 1) {
 			CustomHashtable elementSet = newHashtable(elements.length * 2);
 			for (int i = 0; i < elements.length; i++) {
-				Object element = elements[i];
+				Object element;
+				if(elements[i] instanceof TreeItem){
+					element = ((TreeItem)elements[i]).getData();
+				}else{
+					element = elements[i];
+				}
 				Object old = elementSet.put(element, element);
 				if (old != null) {
 					String message = "Sibling elements in viewer must not be equal:\n  " //$NON-NLS-1$