random occuring NullReferenceException

I have the following problem: My game throws me follwing error at a random occuring basis. I have not found any pattern in it or anything. When I start the game and try to interact with an object sometimes the error pops up. If not the game runs smooth till the finish. I'm quite helpless with it. Has anyone an Idea?

 NullReferenceException
   at (wrapper managed-to-native) UnityEngine.Behaviour:get_isActiveAndEnabled ()
   at UnityEngine.EventSystems.UIBehaviour.IsActive () [0x00002] in C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\EventSystem\UIBehaviour.cs:22 
   at UnityEngine.UI.Graphic.SetVerticesDirty () [0x00002] in C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\UI\Core\Graphic.cs:100 
   at UnityEngine.UI.Text.set_text (System.String value) [0x00053] in C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\UI\Core\Text.cs:138 
   at Score.AddToScore1 (Int32 i, System.String name) [0x000ff] in D:\Unity\TauchroboterSpiel_25_04\Assets\Scripts\Score.cs:43 
   at Collectable.OnEndDrag (UnityEngine.EventSystems.PointerEventData eventData) [0x00188] in D:\Unity\TauchroboterSpiel_25_04\Assets\Scripts\Collectable.cs:166 
   at UnityEngine.EventSystems.ExecuteEvents.Execute (IEndDragHandler handler, UnityEngine.EventSystems.BaseEventData eventData) [0x00008] in C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\EventSystem\ExecuteEvents.cs:80 
   at UnityEngine.EventSystems.ExecuteEvents.Execute[IEndDragHandler] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.EventFunction`1 functor) [0x00073] in C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\EventSystem\ExecuteEvents.cs:269 
 UnityEngine.DebugLogHandler:Internal_LogException(Exception, Object)
 UnityEngine.DebugLogHandler:LogException(Exception, Object)
 UnityEngine.Logger:LogException(Exception, Object)
 UnityEngine.Debug:LogException(Exception)
 UnityEngine.EventSystems.ExecuteEvents:Execute(GameObject, BaseEventData, EventFunction`1) (at C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\EventSystem\ExecuteEvents.cs:273)
 UnityEngine.EventSystems.StandaloneInputModule:ProcessTouchPress(PointerEventData, Boolean, Boolean) (at C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\EventSystem\InputModules\StandaloneInputModule.cs:302)
 UnityEngine.EventSystems.StandaloneInputModule:ProcessTouchEvents() (at C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\EventSystem\InputModules\StandaloneInputModule.cs:200)
 UnityEngine.EventSystems.StandaloneInputModule:Process() (at C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\EventSystem\InputModules\StandaloneInputModule.cs:183)
 UnityEngine.EventSystems.EventSystem:Update() (at C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\EventSystem\EventSystem.cs:287)

Here the related part of the Code (it's a MonoBehaviour):

 public int score1 = 0;
 public int score2 = 0;
 public Text txtscore1, txtscore2;
 
 List<string> collected1 = new List<string>();
 List<string> collected2 = new List<string>();
 
 public OnScreenText onScreenText;

 void Start(){
     
     onScreenText = GameObject.Find("GlobalController").GetComponent<OnScreenText>();
 }
 
 void Awake() {
 DontDestroyOnLoad(transform.gameObject);

}

 public void AddToScore1(int i, string name){
     if(collected1.Count > 0){
         bool add = true;
         foreach(string s in collected1){
             if(s == name){
                 add = false;
             }
         }
         if(add){
             collected1.Add(name);
             score1 += i;
             txtscore1.text = "" + score1;
             onScreenText.DisplayText("+"+i,1f,1);
         }
     }else{
         collected1.Add(name);
         score1 += i;
         txtscore1.text = "" + score1;
         onScreenText.DisplayText("+"+i,1f,1);
     }
 }