Fixing bug 594547, crash downstream from nsTreeContentView::InsertRow(). r=bzbarsky@mit.edu, tnikkel@gmail.com, a=jst@mozilla.org

layout/xul/base/src/tree/src/nsTreeContentView.cpp

@@ -1091,6 +1091,8 @@ nsTreeContentView::ContentInserted(nsIDocument *aDocument,
   }
   else if (childTag == nsGkAtoms::option) {
     PRInt32 parentIndex = FindContent(aContainer);
+
+    if (parentIndex >= 0) {
       PRInt32 index = 0;
       GetIndexInSubtree(aContainer, aChild, &index);
       PRInt32 count = InsertRow(parentIndex, index, aChild);
@@ -1098,6 +1100,7 @@ nsTreeContentView::ContentInserted(nsIDocument *aDocument,
         mBoxObject->RowCountChanged(parentIndex + index + 1, count);
     }
   }
+}
 
 void
 nsTreeContentView::ContentRemoved(nsIDocument *aDocument,