Active users copied to last valid state upon invalidation

dotnet/AipsRT/Model/Whiteboard/Whiteboard.cs

@@ -14,8 +14,7 @@ public class Whiteboard
 
     public List<User> ActiveUsers { get; } = [];
     public void AddActiveUser(User user) => ActiveUsers.Add(user);
-    public void RemoveActiveUser(Guid userId) 
+    public void RemoveActiveUser(Guid userId) => ActiveUsers.RemoveAll(u => u.UserId == userId);
-        => ActiveUsers.RemoveAll(u => u.UserId == userId);
     
     public List<Shape> Shapes { get; } = [];
 
@@ -48,5 +47,11 @@ public class Whiteboard
         TextShapes.Add(shape);
     }
 
-    public void AddUser(User user) => Users.Add(user);
+    public void AddUser(User user)
+    {
+        if (!Users.Contains(user))
+        {
+            Users.Add(user);
+        }
+    }
 }

dotnet/AipsRT/Services/RtErrorHandleStrategy.cs

@@ -18,9 +18,16 @@ public class RtErrorHandleStrategy : IErrorMessageHandleStrategy
     
     public async Task Handle(ErrorMessage message, CancellationToken cancellationToken)
     {
+        var activeUsers = _whiteboardManager.GetWhiteboard(message.WhiteboardId)!.ActiveUsers;
+        
         await _whiteboardManager.LoadWhiteboard(message.WhiteboardId);
         
         var whiteboard = _whiteboardManager.GetWhiteboard(message.WhiteboardId)!;
+
+        foreach (var user in activeUsers)
+        {
+            whiteboard.AddActiveUser(user);
+        }
         
         await _hubContext.Clients
             .Group(whiteboard.WhiteboardId.ToString())