[QGIS-Developer] How do I add a QgsLayoutItemMapGrid to a QgsLayoutItemMap with PyQGIS 3.x?

[Damián Ortega Terol]

I'm trying to add a grid to a map but I don't get any results. My code:

def add_map_layout_item(self,
                        list_maplayers,
                        qgs_rectangle,
                        origin_x_mm,
                        origin_y_mm,
                        width_mm,
                        height_mm,
                        grid_interval):

    map = QgsLayoutItemMap(self.layout)
    map.updateBoundingRect()
    map.setRect(QRectF(origin_x_mm, origin_y_mm, width_mm, height_mm))
    map.setPos(origin_x_mm, origin_y_mm)
    map.setFrameEnabled(True)

    map.setLayers(list_maplayers)
    map.setKeepLayerSet(True)
    map.setExtent(qgs_rectangle)
    self.iface.mapCanvas().freeze(True)

    map.attemptMove(QgsLayoutPoint(origin_x_mm, origin_y_mm))
    map.attemptResize(QgsLayoutSize(width_mm, height_mm))
    map.zoomToExtent(qgs_rectangle)

    qgs_layout_item_map_grid = QgsLayoutItemMapGrid("New grid",
                                                        map)
    qgs_layout_item_map_grid.setAnnotationPrecision(0)
    qgs_layout_item_map_grid.setAnnotationFrameDistance(1)
    qgs_layout_item_map_grid.setAnnotationFontColor(QColor(0, 0, 0))
    qgs_layout_item_map_grid.setCrossLength(1.0)
    qgs_layout_item_map_grid.setIntervalX(grid_interval)
    qgs_layout_item_map_grid.setIntervalY(grid_interval)

    self.layout.addLayoutItem(map)

What am I missing?
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