SerializedProperty mesh, material;

    private void OnEnable()
    {
        mesh = serializedObject.FindProperty("mesh");
        material = serializedObject.FindProperty("material");

    }
    public override void OnInspectorGUI()
    {
        serializedObject.Update();

        Accessory accessory = (Accessory)target;

        EditorGUILayout.PropertyField(mesh);
        EditorGUILayout.PropertyField(material);

        if (GUILayout.Button("Refresh Accessory Mesh"))
        {
            accessory.ChangeAccessory(mesh, material); // this is where the error is
        }

        serializedObject.ApplyModifiedProperties();
    }