Assign a string to the item in listbox

Hi i had just figure out how to do this function but it onli apply to 1 or 2 item. If there many item in the listbox, a run time error will be appear when i click on the edit button(error msg "An unhandled exception of type 'System.ArgumentOutOfRangeException' occurred in mscorlib.dll").

Below is my coding, can anyone please tell me which part of the my code is wrong. thank u

Form main

public ArrayList destination = new ArrayList();
private editForm edit;

public string getDest(){return this.destination[this.listBoxFile.SelectedIndex].ToString();}

public void setDest(string path){this.destination[this.listBoxFile.SelectedIndex] = path;}

public string getCustDest(){return this.destination[this.listBoxFile.SelectedIndex].ToString();}

public void setCustDest(string path){this.destination[this.listBoxFile.SelectedIndex] = path;}

private void buttonEdit_Click(object sender, System.EventArgs e){
            this.destination.Add("Application Directory");
            edit = new editForm(this);
            edit.ShowDialog();
}

Form edit
private void information()
{
    string dest = main.getDest();
    this.comboDest.Text = dest;
    if(dest=="[Custom]")
    {
        this.textDest.Text = main.getCustDest();
    }
    this.textSource.Text = main.link();
}

private void comboDest_SelectedIndexChanged(object sender, System.EventArgs e){
    string select = (string)comboDest.SelectedItem;
    main.setDest(select);
            
    if(select == "[Custom]")
    {
        this.textDest.Enabled = true;
        if(this.textDest.TextLength>0)
        {
            main.setCustDest(this.textDest.Text);
                    
        }
    }
    else
    {
        this.textDest.Enabled = false;
    }
}