Save data to firebase realtime from a picker's item which data is retrieved from firebase also, in xamarin form - firebase-realtime-database

Question

My problem is how to save data from picker's value to firebase realtime. Anyway, Im super beginner to xamarin and so far I cant find any solution to my problem.
This is what I tried.
//my model
`
public int DeliveryPrice { get; set; }
public int PickUp_Price { get; set; }
public string gallonType { get; set; }
public string refillName { get; set; }
public int refill_id { get; set; }
public string waterType { get; set; }
`
//Code how I get the data from my firebase realtime db
`
public async Task<List<PRODUCT_REFILL>> GetAllProductRefill()
{
return
(await firebaseClient.Child
(nameof(PRODUCT_REFILL))
.OnceAsync<PRODUCT_REFILL>()).Select(item => new PRODUCT_REFILL
{
DeliveryPrice=item.Object.DeliveryPrice,
PickUp_Price=item.Object.PickUp_Price,
gallonType=item.Object.gallonType,
refillName=item.Object.refillName,
waterType=item.Object.waterType,
refill_id=item.Object.refill_id
}).ToList();
}
`
//my picker design UI
`
<Picker x:Name="Picker_ProductType"
ItemsSource="{Binding refillName}"
ItemDisplayBinding="{Binding refillName}">
<Picker.Items > <x:String >
</x:String>
<x:String>
</x:String>
</Picker.Items>
</Picker>
`
//my codebehind
`
Product_RefillRepo productrefill = new Product_RefillRepo();
async protected override void OnAppearing()
{
var productRefill = await productrefill.GetAllProductRefill();
Picker_ProductType.ItemsSource = productRefill;
}
`
//in my order button, which is also inside in code behind.cs
string orderProductType = Picker_ProductType.SelectedItem.ToString();
Then if click the order button, I got this error.
Edit; This is the error----- Error says; System.InvalidCastException: 'Specified cast is not valid.'
any link that will be commented that are related to my post, will highly appreciated, Thank you so much.