custom inspector SerializedProperty Class List within a different Class List

I am making a custom inspector for a big script I'm working on. I have a List of a Class TestClass and within that class is another class List of LowerTestClass. Everything works fine until the code tries to run the method LowerShow and I get the error message:

NullReferenceException: Object reference not set to an instance of an object UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at C:/BuildAgent/work/d3d49558e4d408f4/artifacts/EditorGenerated/EditorGUI.cs:5173) UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, UnityEngine.GUILayoutOption[] options) (at C:/BuildAgent/work/d3d49558e4d408f4/artifacts/EditorGenerated/EditorGUI.cs:5159) scriptToEditTest.LowerShow (UnityEditor.SerializedProperty list, System.Collections.Generic.List`1 testclassfunction) (at Assets/Editor/scriptToEditTest.cs:95) scriptToEditTest.Show (UnityEditor.SerializedProperty list, System.Collections.Generic.List`1 testclassfunction) (at Assets/Editor/scriptToEditTest.cs:87) scriptToEditTest.OnInspectorGUI () (at Assets/Editor/scriptToEditTest.cs:41) UnityEditor.InspectorWindow.DrawEditors (Boolean isRepaintEvent, UnityEditor.Editor[] editors, Boolean eyeDropperDirty) (at C:/BuildAgent/work/d3d49558e4d408f4/Editor/Mono/Inspector/InspectorWindow.cs:850) UnityEditor.DockArea:OnGUI()

Script

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class scriptToBeTestedByEditor : MonoBehaviour {
 
     public List <TestClass> testClass;
     public int upperTestInt = 0;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }
 [System.Serializable]
 public class TestClass
 {
     public string className = "";
     public Texture2D image;
     public int testInt = 0;
     public bool showLowerClass = false;
     public List <LowerTestClass> lowerTestClass;
 }
 public class LowerTestClass
 {
     public string lowerClassName = "";
     public Texture2D image;
     public int testInt = 0;
 }

Editor

 using UnityEngine;
 using UnityEditor;
 using System;
 using System.Collections;
 using System.Collections.Generic;
 
 /*[Flags]
 public enum displayFieldType {
     DisplayAsAutomaticFields,
     DisplayAsCustomizableGUIFields
 }*/
 
 [CustomEditor(typeof(scriptToBeTestedByEditor))]
 public class scriptToEditTest : Editor {
 
     //enum displayFieldType {DisplayAsAutomaticFields, DisplayAsCustomizableGUIFields}
     //displayFieldType DisplayFieldType;
 
     int inspectorIndex = 0;
     String[] inspectorOptions = {"DisplayAsAutomaticFields", "DisplayAsCustomizableGUIFields"};
 
     /*SerializedObject GetTarget;
     SerializedProperty ThisList;
     int ListSize ;
 
     void OnEnable(){
         GetTarget = new SerializedObject(target);
         ThisList = GetTarget.FindProperty("testClass"); // Find the List in our script and create a refrence of it
     }*/
     
     //public override 
     public override void OnInspectorGUI()
     {
         serializedObject.Update();
         scriptToBeTestedByEditor testThing = (scriptToBeTestedByEditor)target;
 
         //DisplayFieldType =  EditorGUILayout.EnumPopup("Names of people:", displayFieldType);
         inspectorIndex = EditorGUILayout.Popup(inspectorIndex, inspectorOptions);
         if(inspectorIndex == 1)
         {
             testThing.testClass = Show (serializedObject.FindProperty("testClass"),testThing.testClass);
         }
         else if(inspectorIndex == 0)
         {
             base.OnInspectorGUI();
         }
 
         if(GUI.changed)
         {
             EditorUtility.SetDirty(testThing);
         }
     }
     public static List <TestClass> Show (SerializedProperty list, List <TestClass> testclassfunction) {
         EditorGUILayout.PropertyField(list);
         EditorGUI.indentLevel += 1;
         if (list.isExpanded) {
             //testThing.testClass.Count
             //EditorGUILayout.PropertyField(serializedObject.FindProperty("testClass").FindPropertyRelative("List.Count"));
             int listSize = EditorGUILayout.IntField("list size",testclassfunction.Count);
             if(listSize != testclassfunction.Count && listSize != 0){
                 if(listSize > testclassfunction.Count)
                 {
                     //make bigger
                     int numberOfForLoop = listSize-testclassfunction.Count;
                     for(int i = 0; i < numberOfForLoop; i++)
                     {
                         testclassfunction.Add(new TestClass{});
                     }
                 }
                 else if (listSize < testclassfunction.Count)
                 {
                     //make smaller
                     int numberOfForLoop = testclassfunction.Count - listSize;
                     for(int i = 0; i < numberOfForLoop; i++)
                     {
                         testclassfunction.RemoveAt(testclassfunction.Count-1);
                     }
                 }
             }
             EditorGUI.indentLevel += 1;
             for (int i = 0; i < testclassfunction.Count; i++) {
                 testclassfunction[i].className = EditorGUILayout.TextField("className",testclassfunction[i].className );
                 testclassfunction[i].testInt = EditorGUILayout.IntField("this is int of element: " + i.ToString(),testclassfunction[i].testInt);
                 testclassfunction[i].showLowerClass = EditorGUILayout.Toggle("Show Lower Class?",testclassfunction[i].showLowerClass );
                 if(testclassfunction[i].showLowerClass)
                 {
                     testclassfunction[i].lowerTestClass = LowerShow(list.FindPropertyRelative("lowerTestClass"), testclassfunction[i].lowerTestClass);
                 }
             }
             EditorGUI.indentLevel -= 1;
         }
         EditorGUI.indentLevel -= 1;
         return testclassfunction;
     }public static List <LowerTestClass> LowerShow (SerializedProperty list, List <LowerTestClass> testclassfunction) {
         EditorGUILayout.PropertyField(list);
         EditorGUI.indentLevel += 1;
         if (list.isExpanded) {
             //testThing.testClass.Count
             //EditorGUILayout.PropertyField(serializedObject.FindProperty("testClass").FindPropertyRelative("List.Count"));
             int listSize = EditorGUILayout.IntField("list size",testclassfunction.Count);
             if(listSize != testclassfunction.Count && listSize != 0){
                 if(listSize > testclassfunction.Count)
                 {
                     //make bigger
                     int numberOfForLoop = listSize-testclassfunction.Count;
                     for(int i = 0; i < numberOfForLoop; i++)
                     {
                         testclassfunction.Add(new LowerTestClass{});
                     }
                 }
                 else if (listSize < testclassfunction.Count)
                 {
                     //make smaller
                     int numberOfForLoop = testclassfunction.Count - listSize;
                     for(int i = 0; i < numberOfForLoop; i++)
                     {
                         testclassfunction.RemoveAt(testclassfunction.Count-1);
                     }
                 }
             }
             EditorGUI.indentLevel += 1;
             for (int i = 0; i < testclassfunction.Count; i++) {
                 testclassfunction[i].lowerClassName = EditorGUILayout.TextField("lowerTestClass",testclassfunction[i].lowerClassName );
                 testclassfunction[i].testInt = EditorGUILayout.IntField("this is int of element: " + i.ToString(),testclassfunction[i].testInt);
             }
             EditorGUI.indentLevel -= 1;
         }
         EditorGUI.indentLevel -= 1;
         return testclassfunction;
     }
 
 }