Game Object Created by script disappeared after reopen the project

Hello Guys:
I'm currently writing a game manager, which can be used to edit some basic status of the game character before we put it to the scene. So I'm using the following script to add a character

 public class AddCharacter : EditorWindow
 {
     GameObject obj = null;
     GameObject tempobj = null;
     Vector3 vec3 = new Vector3(0, 0, 0);
     CharacterClass classname = null;
     string objname = "";
     [MenuItem("Asset/TestCreate")]
     public static void ShowWindow()
     {
         var window = GetWindow<AddCharacter>();
         window.title = "Create Character";
     }
     public void OnGUI()
     {
         EditorGUILayout.BeginVertical();
         obj = EditorGUILayout.ObjectField("Select game object to place", obj, typeof(GameObject), true) as GameObject;
         vec3 = EditorGUILayout.Vector3Field("placement position:", vec3);
         objname = EditorGUILayout.TextField("Name for new Character: ", objname);
 
         if (obj && objname != "")
         {
             if (GUILayout.Button("Add to scene"))
             {
                 tempobj = (GameObject)Instantiate(obj, vec3, Quaternion.identity);
                 tempobj.name = objname;
                 Characters temp = tempobj.GetComponent<Characters>();
                 CharacterClass selectedclass = AIManager.Instance.GetNewCharInfo();
                 temp.SetName(objname);
                 temp.SetClass(selectedclass);
                 temp.UpdateCharacter();
                 selectedclass.AddCharacters(objname);
                 AIManager.Instance.setnewChar(true);
                 AIManager.Instance.CreateCharacters(objname);
                 Close();
             }
         }
         else
         {
             EditorGUILayout.LabelField("You must select an object and give it a name first");
         }
         EditorGUILayout.EndVertical();
     }
 }
 

However, I found that everything I've created by using this game manager is disappeared after I reopen my project, so I'm wondering is there a way to keep those changes(Add/delete a game object by script) in Unity?