Thank you kindly, MCortez for a patch that solves:

Different people using Hippo 0.5.1 report that trying 
to send group instant messages crashes the viewer 
(Hippo 0.5.1). This is the case even for empty groups 
or if all group members are online.

1 files changed, 18 insertions, 1 deletions

diff --git a/OpenSim/Region/OptionalModules/Avatar/XmlRpcGroups/XmlRpcGroupsModule.cs b/OpenSim/Region/OptionalModules/Avatar/XmlRpcGroups/XmlRpcGroupsModule.cs
index 5ba7eff..3476cdb 100644
--- a/OpenSim/Region/OptionalModules/Avatar/XmlRpcGroups/XmlRpcGroupsModule.cs
+++ b/OpenSim/Region/OptionalModules/Avatar/XmlRpcGroups/XmlRpcGroupsModule.cs
@@ -1089,6 +1089,11 @@ namespace OpenSim.Region.OptionalModules.Avatar.XmlRpcGroups
 
         private GroupRequestID GetClientGroupRequestID(IClientAPI client)
         {
+            if (client == null)
+            {
+                return new GroupRequestID();
+            }
+
             lock (m_clientRequestIDInfo)
             {
                 if (!m_clientRequestIDInfo.ContainsKey(client.AgentId))
@@ -1098,7 +1103,19 @@ namespace OpenSim.Region.OptionalModules.Avatar.XmlRpcGroups
                     info.RequestID.SessionID = client.SessionId;
 
                     UserProfileData userProfile = m_sceneList[0].CommsManager.UserService.GetUserProfile(client.AgentId);
-                    if (userProfile is ForeignUserProfileData)
+                    if (userProfile == null)
+                    {
+                        // This should be impossible.  If I've been passed a reference to a client
+                        // that client should be registered with the UserService.  So something
+                        // is horribly wrong somewhere.
+
+                        m_log.WarnFormat("[GROUPS]: Could not find a user profile for {0} / {1}", client.Name, client.AgentId);
+
+                        // Default to local user service and hope for the best?
+                        info.RequestID.UserServiceURL = m_sceneList[0].CommsManager.NetworkServersInfo.UserURL;
+
+                    }
+                    else if (userProfile is ForeignUserProfileData)
                     {
                         // They aren't from around here
                         ForeignUserProfileData fupd = (ForeignUserProfileData)userProfile;