Replace List Items with New Ones

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C#

ListBox

I am building a small application that allows users to view different types of weeds, flowers and vegetables, I have 3 class Weeds, Flowers and Vegetables.
I would like to clear the list_collection Weed Collection and replace with the Vegetables collection once the GetVegetablesCollection method has been called.

Weeds[] cWeeds = new Weeds[3];
Vegatables[] cVegatables = new Vegatables[2];

private void GetWeedCollection()
{
    cWeeds[0] = new Weeds("Chicory", 100, 47);
    cWeeds[1] = new Weeds("Chinese Cabbage", 105, 37);
    cWeeds[2] = new Weeds("Cress", 81, 76);

    foreach (Weeds x in cWeeds)

    {list_collection.Items.Add(x.Name.ToString());}
}
private void GetVegatablesCollection()
{
    cVegatables[0] = new Vegatables("Cucumber", 5, 3);
    cVegatables[1] = new Vegatables("Carrot", 10, 15);

    foreach (Vegatables x in cVegatables)

    { list_collection.Items.Add(x.Name.ToString()); }

The problem I get is when I call the method, I receive an 'Object reference not set to an instance of an object' on the SelectedItem property.

private void list_collection_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
        if (list_collection.SelectedItem.ToString() == cWeeds[0].Name)
    {
        txt_name.Text = cWeeds[0].Name.ToString();
        txt_cal.Text = cWeeds[0].Calcium.ToString() + "mg";
        txt_phos.Text = cWeeds[0].Phosphrous.ToString() + "mg";
    }

    if (list_collection.SelectedItem.ToString() == cVegatables[0].Name)
    {
        txt_name.Text = cVegatables[0].Name.ToString();
        txt_cal.Text = cVegatables[0].Calcium.ToString() + "mg";
        txt_phos.Text = cVegatables[0].Phosphrous.ToString() + "mg";
    }

Here is the event that calls the method;

private void btn_VegatablesChoice_Click(object sender, RoutedEventArgs e)

        { list_collection.Items.Clear(); GetVegatablesCollection(); }

Hope this made sense, thank you.