Add an Object to a List which is part of an array does not work.

I have the problem, that i want to add objects to a list which is part of an array. i was searching in the forum, but could not find a correct solution, because mostly the problem was in the instantiating of the list and that is what i have done correct(as far as i can say):

the error comes in line 27 of the 2nd part of code.

so here is the part of my code that makes the problem:

  public class PlaygroundCreatorWorkingConcept : MonoBehaviour
  {
      public int height;
      public int width;
     
      List<WallInformation> WallInformations = new List<WallInformation>();
      List<WallInformation>[,] WallInformationSmall;
  }
  
   public class WallInformation
      {
          public Coord coordinate;
          public bool isWall;
          public float floorHeight;
          public float ceilingHeight;
  
          public WallInformation(Coord newCoordinate, bool newIsWall, float newFloorHeight, float newCeilingHeight)
          {
              coordinate = newCoordinate;
              isWall = newIsWall;
              floorHeight = newFloorHeight;
              ceilingHeight = newCeilingHeight;
          }
      }
  
  
  void Awake()
      {
     WallInformationSmall = new List<WallInformation>[width / 10, height / 10];
          for(int i = 0; i >= width / 10; i++)
          {
              for (int j = 0; j >= height / 10; j++)
              {
                  WallInformationSmall[i, j] = new List<WallInformation>();
              }
          }
       }

this is the top of the programm with all important thing for the question

   public void SetWallInformations()
      {
          for (int x = 0; x < width; x++)
          {
              for (int z = 0; z < height; z++)
              {
                  WallInformations.Add(new WallInformation(new Coord(x, z), isWall[x, z], floorWorld[x, z], ceilingWorld[x, z]));
              }
          } //scheint bis hier zu funktionieren, da er bei small 50*50 = 2500 informationen füllt.
          foreach (WallInformation WallInformation in WallInformations)
          {
              Debug.Log(WallInformations.Count);
              Debug.Log(WallInformation.coordinate.tileX);
              Debug.Log(WallInformation.coordinate.tileZ);
              Debug.Log(WallInformation.isWall);
              Debug.Log(WallInformation.floorHeight);
              Debug.Log(WallInformation.ceilingHeight);
              int i = 0;
              int j = 0;
              if (WallInformation.coordinate.tileX >= i + 0 && WallInformation.coordinate.tileX <= i + 10)
              {
                  Debug.Log("Bin Hier1.");
  
                  if (WallInformation.coordinate.tileZ >= j + 0 && WallInformation.coordinate.tileZ <= j + 10)
                  {
                      Debug.Log("Bin Hier2.");
                      WallInformationSmall[i, j].Add(WallInformation);
                  }
                  else
                  {
                      j++;
                      //return;
                      Debug.Log("Bin Hier3.");
                  }
              }
              else
              {
                  i++;
                  j = 0;
                  //return;
              }
              i = 0;
              Debug.Log("Bin Hier4.");
  
          }
     }

the problem is that the line below Debug.Log("Bin Hier2"); does give me this error:

NullReferenceException: Object reference not set to an instance of an object