Null Reference Exception

I want to write an Settings menu and I already created the UI but when I click on a button I get a null reference exception.

NullReferenceException: Object reference not set to an instance of an object SettingsManager.SetResolutionHeight (Int32 height) (at Assets/Scripts/Options/SettingsManager.cs:36) UnityEngine.Events.InvokableCall`1[System.Int32].Invoke (System.Object[] args) (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:189) UnityEngine.Events.CachedInvokableCall`1[System.Int32].Invoke (System.Object[] args) (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:309) UnityEngine.Events.InvokableCallList.Invoke (System.Object[] parameters) (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:637) UnityEngine.Events.UnityEventBase.Invoke (System.Object[] parameters) (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:773) UnityEngine.Events.UnityEvent.Invoke () (at C:/buildslave/unity/build/Runtime/Export/UnityEvent_0.cs:52) UnityEngine.UI.Button.Press () (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:35) UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:44) UnityEngine.EventSystems.ExecuteEvents.Execute (IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:50) UnityEngine.EventSystems.ExecuteEvents.Execute[IPointerClickHandler] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.EventFunction`1 functor) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:261) UnityEngine.EventSystems.EventSystem:Update()

Here is my Code

This is my data class:

 public class Options_data
 {
     public int width, height;
     public bool fullscreen;
     public int textureQuality, antialiasing, vSync, shadowType, shadowResolution;
     public float masterVolume, musicVolume, surroundingVolume, battleVolume;
 }

This is the class where I Change and set the values:

 using UnityEngine;
 using UnityEngine.UI;
 
 public class SettingsManager : MonoBehaviour {
 
     private Options_data data;
     public Slider[] musicSlider;
     public Toggle fullScreenToggle;
 
     private bool fullscreen = true;
 
     private int screenWidth = 1920, screenHeight = 1080;
 
     private void Start()
     {
         fullScreenToggle.onValueChanged.AddListener(delegate { SetFullscreen(); });
     }
 
     public void SetTextureQuality(int idx)
     {
         data.textureQuality = QualitySettings.masterTextureLimit = idx;
     }
 
     public void SetResolution()
     {
         Screen.SetResolution(screenWidth, screenHeight, fullscreen, 60);
     }
 
     public void SetResolutionWidth(int width)
     {
         screenWidth = data.width = width;
     }
 
     public void SetResolutionHeight(int height)
     {
         screenHeight = data.height = height;
     }
 
     public void SetShadowType(int idx)
     {
 
     }
 
     public void SetShadowResolution(int idx)
     {
 
     }
 
     public void SetFullscreen()
     {
         data.fullscreen = fullscreen = fullScreenToggle.isOn;
     }
 
     public void SetAntiAliasing(int idx)
     {
         data.antialiasing = QualitySettings.antiAliasing = idx;
     }
 
     public void SetVSync(int idx)
     {
 
     }
 
     public void Apply()
     {
 
     }
 
     public void Revert()
     {
 
     }
 
     private void Update()
     {
         
     }
 }

The Code is not finished

Thanks for any help :)