GUI Editior Script not working in ( Unity 2019.x )

Hi, so I was following This Flocking algorithm tutorial and everything worked perfectly till i got to this episode of the tutorial which is about making A custom editor mini extention type thing...(I dont know how to describe the epesode), after following it, i found out that most of the script doesn't work, and thats not a mistake in the actual script, its a problem that has got to do with unity versions, and the script doesnt work in the 2019 versions of unity for some reason, i found an explenation of how to work around that in the comments which is posted by " Dexter André Osiander " but i cant seem to be able to preform it, I've never worked with the GUI system before and i mostly dont understand the script, i am sure its simple but i cant get figure it out for some reason, so what i am asking for is can someone follow the instructions of this guy's comment or come up with a solution himself, any help is appreciated. Thanks in advance!

here is the script :

using System.Collections;

using System.Collections.Generic;

using UnityEngine;

using UnityEditor;

using UnityEngine.UIElements;

[CustomEditor(typeof(CompositeBehavior))]

public class CompositeBehaviorEditor : Editor

{

 public override void OnInspectorGUI()
 {
     //setup
     CompositeBehavior cb = (CompositeBehavior)target;

     Rect r = EditorGUILayout.BeginHorizontal();
     r.height = EditorGUIUtility.singleLineHeight;

     //check for behaviors
     if (cb.behaviors == null || cb.behaviors.Length == 0)
     {
         EditorGUILayout.HelpBox("No behaviors in array.", MessageType.Warning);
         EditorGUILayout.EndHorizontal();
         r = EditorGUILayout.BeginHorizontal();
         r.height = EditorGUIUtility.singleLineHeight;
     }
     else
     {
         r.x = 30f;
         r.width = EditorGUIUtility.currentViewWidth - 95f;
         EditorGUI.LabelField(r, "Behaviors");
         r.x = EditorGUIUtility.currentViewWidth - 65f;
         r.width = 60f;
         EditorGUI.LabelField(r, "Weights");
         r.y += EditorGUIUtility.singleLineHeight * 1.2f;

         EditorGUI.BeginChangeCheck();
         for (int i = 0; i < cb.behaviors.Length; i++)
         {
             r.x = 5f;
             r.width = 20f;
             EditorGUI.LabelField(r, i.ToString());
             r.x = 30f;
             r.width = EditorGUIUtility.currentViewWidth - 95f;
             cb.behaviors[i] = (FlockBehavior)EditorGUI.ObjectField(r, cb.behaviors[i], typeof(FlockBehavior), false);
             r.x = EditorGUIUtility.currentViewWidth - 65f;
             r.width = 60f;
             cb.weights[i] = EditorGUI.FloatField(r, cb.weights[i]);
             r.y += EditorGUIUtility.singleLineHeight * 1.1f;
         }
         if (EditorGUI.EndChangeCheck())
         {
             EditorUtility.SetDirty(cb);
         }
     }

     EditorGUILayout.EndHorizontal();
     r.x = 5f;
     r.width = EditorGUIUtility.currentViewWidth - 10f;
     r.y += EditorGUIUtility.singleLineHeight * 0.5f;
     if (GUI.Button(r, "Add Behavior"))
     {
         AddBehavior(cb);
         EditorUtility.SetDirty(cb);
     }

     r.y += EditorGUIUtility.singleLineHeight * 1.5f;
     if (cb.behaviors != null && cb.behaviors.Length > 0)
     {
         if (GUI.Button(r, "Remove Behavior"))
         {
             RemoveBehavior(cb);
             EditorUtility.SetDirty(cb);
         }
     }


 }

 void AddBehavior(CompositeBehavior cb)
 {
     int oldCount = (cb.behaviors != null) ? cb.behaviors.Length : 0;
     FlockBehavior[] newBehaviors = new FlockBehavior[oldCount + 1];
     float[] newWeights = new float[oldCount + 1];
     for (int i = 0; i < oldCount; i++)
     {
         newBehaviors[i] = cb.behaviors[i];
         newWeights[i] = cb.weights[i];
     }
     newWeights[oldCount] = 1f;
     cb.behaviors = newBehaviors;
     cb.weights = newWeights;
 }

 void RemoveBehavior(CompositeBehavior cb)
 {
     int oldCount = cb.behaviors.Length;
     if (oldCount == 1)
     {
         cb.behaviors = null;
         cb.weights = null;
         return;
     }
     FlockBehavior[] newBehaviors = new FlockBehavior[oldCount - 1];
     float[] newWeights = new float[oldCount - 1];
     for (int i = 0; i < oldCount - 1; i++)
     {
         newBehaviors[i] = cb.behaviors[i];
         newWeights[i] = cb.weights[i];
     }
     cb.behaviors = newBehaviors;
     cb.weights = newWeights;
 }

}