LimitToFirst in Firebase (unity)

Currently an application is being developed in Unity where I need to get the top 3 products in use. I am using the following code when fetching data:

FirebaseDatabase.DefaultInstance
        .GetReference ("Products").OrderByChild("used").LimitToFirst(3)
        .ValueChanged += HandleValueChanged;
    }

    void HandleValueChanged(object sender, ValueChangedEventArgs args) {
    if (args.DatabaseError != null) {
        Debug.LogError (args.DatabaseError.Message);
        return;
    }

      

        
        
        
      

    

It doesn't work, I get this error:

System.NullReferenceException: Object reference not set to an instance of an object at ShowProducts.HandleValueChanged (System.Object sender, Firebase.Database.ValueChangedEventArgs args) [0x000be] in C:\Users\jorren\Documents\JKL\Assets\Scripts\ShowProducts.cs:697 at Firebase.Database.Internal.Core.ValueEventRegistration.FireEvent (Firebase.Database.Internal.Core.View.DataEvent eventData) [0x00000] in <filename unknown>:0 at Firebase.Database.Internal.Core.View.DataEvent.Fire () [0x00000] in <filename unknown>:0 at Firebase.Database.Internal.Core.View.EventRaiser+Runnable30.Run () [0x00000] in <filename unknown>:0 at Firebase.Database.DotNet.DotNetPlatform+SynchronizationContextTarget+<PostEvent>c__AnonStorey0.<>m__0 (System.Object x) [0x00000] in <filename unknown>:0 at Firebase.Unity.UnitySynchronizationContext+SynchronizationContextBehavoir+<Start>c__Iterator0.MoveNext () [0x00000] in <filename unknown>:0 UnityEngine.Debug:Log(Object) Firebase.Unity.<Start>c__Iterator0:MoveNext() UnityEngine.SetupCoroutine:InvokeMoveNext(IEnumerator, IntPtr)

But when I use LimitToLast () it works fine

Many thanks!