Game freezes, cannot see why

Hello all My game freezes randomly, mostly when changing from one level to the next. I cannot tell why this is the case. There are no errors or warnings. This happens when i build the game and when I am testing the game in the Unity editor. It will say unfreeze itself after a few seconds. I have attached one of the level codes and the timer code. I have also attached a picture of how it looks when it freezes.

This is one of the level codes:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using System.IO.Ports;
 public class Arduino5 : MonoBehaviour
 {
 
     SerialPort sp = new SerialPort("\\\\.\\COM5", 9600);// Com port and the baud rate of the arduino
     Material m_Material;
     GameObject Sphere;
 
 
 
     void Awake()
     {
         Sphere = GameObject.FindWithTag("Player");
         m_Material = GameObject.FindWithTag("Player").GetComponent<Renderer>().material;
     }
     void Start()
     {
         if (!sp.IsOpen)
         { // If the erial port is not open 
             sp.Open(); // Open 
         }
         sp.ReadTimeout = 250; // Timeout for reading 
     }
 
     // Update is called once per frame
     void Update()
     {
         if (sp.IsOpen)
         { // Check to see if the serial port is open 
             try
             {
 
                 string portreading = sp.ReadLine(); // get the string output of the serial port 
                 float amount = int.Parse(portreading);
 
 
 
                 if ((amount > 50f) )
                 {
                     m_Material.color = Color.grey;
                     Renderer _rend = Sphere.GetComponent<Renderer>();
                     Sphere.GetComponent<Renderer>().material = _rend.material;
                     _rend.material.mainTexture = Resources.Load("face4") as Texture;
 
 
                 }
                 else
                 {
                     m_Material.color = Color.white;
                 }
             }
             catch (System.Exception)
             {
 
             }
 
         }
 
     }
 }

This is the timer code:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 using UnityEngine.SceneManagement;
 
 public class Timer : MonoBehaviour
 {
     public int timeLeft = 5;
     public Text countdownText;
 
     // Use this for initialization
     void Start()
     {
         StartCoroutine("LoseTime");
     }
 
     // Update is called once per frame
     void Update()
     {
         countdownText.text = ("Time Left = " + timeLeft);
 
         if (timeLeft <= 0)
         {
             StopCoroutine("LoseTime");
             countdownText.text = "Times Up!";
             Invoke("ChangeLevel", 1.0f);
         }
         
     }
 
     IEnumerator LoseTime()
     {
         while (true)
         {
             yield return new WaitForSeconds(1);
             timeLeft--;
         }
     }
     void ChangeLevel()
     {
 
         SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex + 1);
     }
 
 }