I try to study UniRX. And I write the simple script for controlling player:
'''
using UnityEngine;
using UniRx;
using UniRx.Triggers;
namespace playerV1
{
    public class Inputs : MonoBehaviour
    {
        public static Inputs Instance { get; private set; }
        public ReadOnlyReactiveProperty<Vector2> ImpulseForce { get; private set; }
        private void Awake()
        {
            Instance = this;
            ImpulseForce = this.FixedUpdateAsObservable()
                .Select
                (
                _ =>
                {
                    float x = Input.GetAxis("HorizontalX");
                    float z = Input.GetAxis("HorizontalZ");
                    return new Vector2(x, z).normalized;
                }
                )
                .ToReadOnlyReactiveProperty();
        }
    }
    
    public class PlayerController : MonoBehaviour
    {
        public float coefficient;
        private Rigidbody _rg;
        private void Awake()
        {
            _rg = GetComponent<Rigidbody>();
        }
        private void Start()
        {
            Inputs inputs = Inputs.Instance;
            //Debug.Log(inputs.ImpulseForce); I get the mistake here
            inputs.ImpulseForce
                .Where(force => force != Vector2.zero)
                .Subscribe
                (
                force =>
                {
                    _rg.AddForce(coefficient * new Vector3(force.x, 0, force.y), ForceMode.Impulse);
                }
                );
        }
    }
}
'''
But I get mistake: "NullReferenceException: Object reference not set to an instance of an object playerV1.PlayerController.Start () (at Assets/Scripts/PlayerController.cs:43)" I don't understand what is wrong here. What is problem might be here?
 
    