Deep copy the collection of at_target objects so it can't be modified while

it's being iterated

OpenSim/Region/Framework/Scenes/Scene.cs

@@ -1666,7 +1666,10 @@ namespace OpenSim.Region.Framework.Scenes
         {
             List<SceneObjectGroup> objs = new List<SceneObjectGroup>();
             lock (m_groupsWithTargets)
-                objs = new List<SceneObjectGroup>(m_groupsWithTargets.Values);
+            {
+                foreach (SceneObjectGroup grp in m_groupsWithTargets.Values)
+                    objs.Add(grp);
+            }
 
             foreach (SceneObjectGroup entry in objs)
                 entry.checkAtTargets();