What causes the NullReferenceException in Editor Window code

I have editor window where after loading some prefabs I put created game object out of prefabs into scene. For placing I use button where I register event dusringSceneGui with my function. Everything works just fine, but console gives me an NullReferenceException error after placing the object:

 NullReferenceException
 UnityEditor.SceneView.DoOnGUI () (at <9de32d58e117474d9df95577a7ba5c6d>:0)
 UnityEditor.SceneView.OnSceneGUI () (at <9de32d58e117474d9df95577a7ba5c6d>:0)
 UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize, System.Action onGUIHandler, System.Boolean canAffectFocus) (at <d251562a4ccd481bac59212472bdabeb>:0)  

This error is much longer but I find it useless to put here the whole message. Unfortunately there is no back tracing available, so I am not able to tell what causes this error. I suspect the SceneView.duringSceneGui might be culrpit, since error is SceneViewDoOnGUI(). Code snippet of main functions:

 private void OnGUI()
  {
             // some nasty gui here
 
             if (GUILayout.Button("Button"))
             {
                 SceneView.duringSceneGui += PlaceObject;
             }
 }
 
  void PlaceObject(SceneView sceneview)
         {
             HandleUtility.AddDefaultControl(GUIUtility.GetControlID(FocusType.Passive));
             Event e = Event.current;
             Ray ray = HandleUtility.GUIPointToWorldRay(e.mousePosition);
             RaycastHit hit;
 
             if (e.type == EventType.MouseDown && e.button == 0)
             {
                 if (Physics.Raycast(ray, out hit, 200f))
                 {
                     // Placing wokring prefab here, until here everything works
                     SceneView.duringSceneGui -= PlaceObject;
                 }
             }
 }