Instantiating objects only after a scene is loaded

I'm trying to spawn in objects into the scene from a save file. All the logic is done through my "GameManager" script which is a singleton and persists across both scenes. When transitioning from my main menu to the main level, I call

 SceneManager.LoadScene("main_level");

However, this totally screws up my entire saving and loading methods. Apparently me methods get called before the scene loads, and hence the loadscene function resets my entire scene which throws a bunch of errors. How do I resolve this?

 public void LoadGame()
     {
         SceneManager.LoadScene("main_level");

         string jsonData;
         if (newGame) // Start new game
         {
             var defaultPath = Application.streamingAssetsPath + "/Default/Default.json";
             if (File.Exists(defaultPath))
             {
                 jsonData = File.ReadAllText(defaultPath);
                 gameData = JsonUtility.FromJson<GameData>(jsonData);
             }
             else
             {
                 Debug.LogError("Save file is missing at: " + defaultPath);
             }
         }
         else if (File.Exists(path + "/" + saveName))
         {
             jsonData = File.ReadAllText(path + "/" + saveName);
             gameData = JsonUtility.FromJson<GameData>(jsonData);
         }
         else
         {
             Debug.LogError("Save file is missing at: " + path + "/" + saveName);
         }
  
         /* Instantiate required objects */
  
         // Trees
         var treeParent = new GameObject("Trees");
         foreach (TreeSaveLoad.TreeData tree in gameData.trees)
         {
             var treeType = oakTree;
             switch (tree.type)
             {
                 case (TreeSaveLoad.TreeType.Redwood):
                     treeType = redwoodTree;
                     break;
                 case (TreeSaveLoad.TreeType.Birch):
                     treeType = birchTree;
                     break;
                 case (TreeSaveLoad.TreeType.Beech):
                     treeType = beechTree;
                     break;
                 case (TreeSaveLoad.TreeType.Pine):
                     treeType = pineTree;
                     break;
                 case (TreeSaveLoad.TreeType.Oak):
                     treeType = oakTree;
                     break;
                 case (TreeSaveLoad.TreeType.Maple):
                     treeType = mapleTree;
                     break;
                 default:
                     Debug.LogError("Missing tree type.");
                     break;
             }
             var treeChild = Instantiate(treeType, tree.position, Quaternion.identity) as Transform;
             treeChild.SetParent(treeParent.transform, true);
             yield return null;
         }
  
         // Call load event
         if (EventManager.Instance.e_loadGame != null)
         {
             EventManager.Instance.e_loadGame.Invoke();
         }
  
         // Call last, after loading is finished
         if (EventManager.Instance.e_loadedGame != null)
         {
             EventManager.Instance.e_loadedGame.Invoke();
         }

Here is the full code for reference. I call all my methods and events AFTER the loadscene call. This script exists on the GameManager object which persists from the main menu to the new scene.