Hi people, I'm trying to display a progress bar when I calculate some things in the Inspector but it appears this message:

ArgumentException: You can only call GUI functions from inside OnGUI.
UnityEngine.GUIUtility.CheckOnGUI () 
UnityEngine.GUILayoutUtility.DoGetRect (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) [0x00000]
UnityEngine.GUILayoutUtility.GetRect (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) [0x00000]
UnityEditor.InspectorWindow.OnGUI () 
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000]

The main code of the example is the follow, this is the editor of another class ViewArea:

using UnityEditor;
using UnityEngine;

[CustomEditor(typeof(ViewArea))]
public class ViewAreaEditor : Editor{

    public override void OnInspectorGUI()
    {
        ViewArea va;
        va = target as ViewArea;

        EditorGUILayout.BeginHorizontal();
        EditorGUILayout.PrefixLabel("Cell Division");
        va.cellDivision = EditorGUILayout.Slider(va.cellDivision, 0.01f, 1f);
        EditorGUILayout.EndHorizontal();

        EditorGUILayout.BeginHorizontal();
        EditorGUILayout.PrefixLabel("Ray Division");
        va.rayDivision = EditorGUILayout.Slider(va.rayDivision, 0.01f, 1f);
        EditorGUILayout.EndHorizontal();

        EditorGUILayout.BeginHorizontal();
        EditorGUILayout.LabelField("Number Of Rays", va.numberOfRays.ToString());
        EditorGUILayout.EndHorizontal();

        if (GUILayout.Button("Calculate PVS"))
        {
            EditorUtility.DisplayProgressBar("Calculating the PVS", "Calculating - 0%", 0.0f);
            va.ThrowRays();
            EditorUtility.DisplayProgressBar("Calculating the PVS", "Calculating - 50%", 0.5f);
            va.CalculateTransformsPerCell();
            EditorUtility.DisplayProgressBar("Calculating the PVS", "Calculating - 100%", 1.0f);
            EditorUtility.ClearProgressBar();
        }
        if (GUI.changed)
        {
            EditorUtility.SetDirty(va);
        }

    }
}