ArgumentException: Getting control 0's position in a group with only 0 controls when doing Repaint

Hi, what in this code is wrong to cause the following error?

   if (fileList.Length > 0 && currentCharacter != null)
   {
     if(listOfPlayers.Count > 1)
     {
       // if server, show level picker
       if(Network.isServer)
       {
             int smallestLevel = 999999;
      
         // loop trough player list
         foreach(NetworkUser user in listfPlayers)
         {
           // get smallest characterLevel number
            smallestLevel = Math.Min(smallestLevel,
           user.characterLevel);
         }
      
         for (int i = 0; i < smallestLevel; i++)
         {
           Debug.Log (selectedLevelIndex + " - " + i + " - " + levelList[i].Number + ": " +
      levelList[i].Name);
 
           if (GUILayout.Toggle(selectedLevelIndex == i, levelList[i].Number + ": " + levelList[i].Name, "toggle"))
           {
              selectedLevelIndex = i;
           }
         }
       }
     }
     else
     {
       selectedLevelIndex = 0;
     }
   }

error on row 23:

ArgumentException: Getting control 0's position in a group with only 0 controls when doing Repaint

This code is ran inside the OnGUI.