Thank you kindly, CMickeyb for a patch that:

Moved intialization of appearance from the SendInitialData 
event handler into CompleteMovement handler. That ensures 
that m_appearance is initialized before the inventory is 
retrieved (so there is a place to check on attachments).

OpenSim/Data/MySQL/MySQLUserData.cs

@@ -722,6 +722,12 @@ namespace OpenSim.Data.MySQL
                 reader.Dispose();
                 result.Dispose();
 
+                if (null == appearance)
+                {
+                    m_log.WarnFormat("[USER DB] No appearance found for user {0}", user.ToString());
+                    return null;
+                }
+                
                 appearance.SetAttachments(GetUserAttachments(user));
 
                 return appearance;