Dijkstra Algorithm, ArgumentNullException: Argument cannot be null.

Hi I am trying to implement Dijkstras Algorithm as a basic pathfinding mathod on an chessboard like grid. I want place a block at every tile I used for a path between two tiles given by the user. I got it working so far, but when I try to use every tile just once, so it is blocked for every future path, I get this Error:

 ArgumentNullException: Argument cannot be null.
 Parameter name: key
 System.Collections.Generic.Dictionary`2[UnityEngine.GameObject,System.Single].get_Item (UnityEngine.GameObject key) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:136)
 DijkstraAlgorithm.Dijkstra (UnityEngine.GameObject[,] grid, UnityEngine.GameObject start, UnityEngine.GameObject end) (at Assets/Scripts/DijkstraAlgorithm.cs:50)
 MouseScript.Update () (at Assets/Scripts/MouseScript.cs:41)

This would be my code:

 static public Stack<GameObject> Dijkstra(GameObject[,]grid, GameObject start, GameObject end)
     {
         Dictionary<GameObject, float> dist = new Dictionary<GameObject, float>();
         Dictionary<GameObject, GameObject> previous = new Dictionary<GameObject, GameObject>();
         List<GameObject> Q = new List<GameObject>();
         
         foreach(GameObject v in grid)
         {
             dist[v] = Mathf.Infinity;
             previous[v] = null;
             Q.Add(v);
         }
 
         dist[start] = 0;
         while(Q.Count > 0)
         {
             float shortestDistance = Mathf.Infinity;
             GameObject shortestDistanceNode = null;
             foreach(GameObject obj in Q)
             {
                 if (dist[obj] < shortestDistance )
                 {
                     shortestDistance = dist[obj];
                     shortestDistanceNode = obj;
                 }
                 
             }
 
             GameObject u = shortestDistanceNode;
 
             Q.Remove(u);
             if(u==end)
             {
                 Stack<GameObject> stack = new Stack<GameObject>();
                 while(previous[u] != null)
                 {
                     stack.Push(u);
                     u = previous[u];
                 }
                 return stack;
             }
 
             if(dist[u] == Mathf.Infinity)
             {
                 break;
             }
 
             foreach(GameObject v in u.GetComponent<Node>().neighbors)
             {
                 if(v.GetComponent<Node>().getBlocked()==false)
                 {
                     break;
                 }
                 float alt = dist[u] + (u.transform.position - v.transform.position).magnitude;
                 if(alt< dist[v])
                 {
                     dist[v] = alt;
                     previous[v] = u;
                 }
             }
 
         }
         return null;
         
     }
 }

I hope someone here can help me, googleing the error has not produced anything helpfull. Thank you in advance Greets Harker