NetworkVariableBase.MarkNetworkBehaviourDirty () Why this error?

public class Generate : NetworkBehaviour
{
    private NetworkVariable<float> decreament = new NetworkVariable<float>(10);
    // Update is called once per frame
    void Update()
    {
        decreament.Value -= Time.deltaTime;
    }
}

Can anyone describe please why this code give me this error after I connect the host and directly close the game from unity?

NullReferenceException: Object reference not set to an instance of an object
Unity.Netcode.NetworkVariableBase.MarkNetworkBehaviourDirty ()