Adding to a text file. It works but I get this message afterwoods

Hi guys/girls! I'm trying to write to a text file and I keep getting this error message.

 IOException: Sharing violation on path D:\githubStuff\Assets\HighScores.txt
 System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.IO/FileStream.cs:320)
 System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share)
 (wrapper remoting-invoke-with-check) System.IO.FileStream:.ctor (string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare)
 System.IO.StreamWriter..ctor (System.String path, Boolean append, System.Text.Encoding encoding, Int32 bufferSize) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.IO/StreamWriter.cs:124)
 System.IO.StreamWriter..ctor (System.String path)
 (wrapper remoting-invoke-with-check) System.IO.StreamWriter:.ctor (string)
 GameManager.Awake () (at Assets/Scripts/GameManager.cs:50)

It does write to the file but I'm not sure why this message pops up.

Here is my Code

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using System.IO;
 
 public class GameManager : MonoBehaviour {
     public static GameManager instance = null;
 
     private TowerManager tower;
     private DropperCamera camera;
     private CameraFollow playerCamera;
     private GameObject player;
     private GameObject blobbi;
     private GameObject shadow;
     private StreamWriter sw;
 
     // Are we using a controller or KB/mouse?
     public bool controller = false;
 
     void OnLevelWasLoaded(int level) {
         tower = GameObject.Find("Tower").GetComponent<TowerManager>();
         // TODO: DON'T HARDCODE YOUR NAMES NICK YOU MORON
         camera = GameObject.Find("IsoCamera").GetComponent<DropperCamera>();
         playerCamera = GameObject.Find("Main Camera").GetComponent<CameraFollow>();
         player = GameObject.Find("Player");
         blobbi = GameObject.Find("Blobbi");
         shadow = GameObject.Find("ShadowGoo");
         // Ignore physical collisions between the blocks and the world
         // We're doing this manually instead
         Physics.IgnoreLayerCollision(0, 8);
         // Also blocks and other blocks
         Physics.IgnoreLayerCollision(8, 8);
         
     } 
 
     // Use this for initialization
     void Awake () {
         OnLevelWasLoaded (1);
         // Singleton stuff - there can only be one
         if (instance == null)
         {
             instance = this;
         } else if (instance != this) {
             Destroy(instance);    
         }
         
         DontDestroyOnLoad(instance);
         sw = new StreamWriter ("Assets/HighScores.txt");
     }
 
     public GameObject getPlayer()
     {
         return player;
     }
 
     void RestartLevel()
     {    
         string line = (getPlayer ().transform.position.y / 2).ToString ();
         SaveHighScore (line);
         Application.LoadLevel (Application.loadedLevelName);
     }
 
     // Update is called once per frame
     void Update () {
 
 
         if (Input.GetKey (KeyCode.Return))
         {
             RestartLevel();
         }
     }
 
     void SaveHighScore(string line){
         sw.WriteLine (line);
         sw.Flush();
     }