Set the property of type item to null value if the item is null

I am setting a property of type item using the setPropertyItem from JavaScript code as below:
var Item = innovator.newItem("Material", "get");
Item.setAttribute("select","id");
Item.setProperty("id", gridRow.entity.ExistingVariantMaterialId);
Item =Item.apply()
if(Item!==null){
partVariantItem.setPropertyItem("material", Item);
}
Now if i want to reset this property to null value if the item is null ,so how can i do that,please help