C# object reference and file already in use

Question

I'm trying to make a simple console program to read and write to a text file, which I found on the internet. But there are several object-reference errors that hinder me and i can't seem to find a solution or satisfying answer to my problem. There is also an error partaining the text file already being in us by another process. They are located between //--------------- in the code. Please advice

namespace SavingNames
{
class Program
{
    public static void Main(string[] args)
    {
        Naam[] personen = new Naam[1];
        int C;

        do
        {
            //menu
            Console.WriteLine("1. Load data.");
            Console.WriteLine("2. Display persons.");
            Console.WriteLine("3. Add new Person.");
            Console.WriteLine("4. Exit.");
            Console.WriteLine("Make a choice 1-4: ");
            C = Convert.ToInt32(Console.ReadLine());

            switch (C)
            {
                case 1:
                    GetData(ref personen);

                    break;
                case 2:
                    ShowAll(personen);
                    break;
                case 3:
                    AddPerson(ref personen);
                    break;
                case 4:
                    break;
            }
        } while (C != 4);

    }


    public static void GetData(ref Naam[] personen)
    {
        StreamReader reader = new StreamReader("Names.txt");
        int size = 0;

        using (reader = File.OpenText("Names.txt"))
        {
            while (reader.ReadLine() != null)
            {
                size++;
            }
        }

        personen = new Naam[size];
        reader = File.OpenText("Names.txt");

        for (int i = 1; i < personen.Length; i++)
        {
            personen[i] = new Naam();
            personen[i].Name = reader.ReadLine();
            personen[i].Livingplace = reader.ReadLine();
            personen[i].Profession = reader.ReadLine();
        }

        reader.Close();

        Console.WriteLine("Data loaded. Press any key to continue...");
        Console.ReadKey();
        Console.Clear();
    }

    public static void ShowAll(Naam[] personen)
    {
        Console.Clear();

        for (int i = 0; i < personen.Length; i++)
        {
           //---------------------------------------------------
           personen[i].Display();//error object-reference-not-set-to-an-instance-error happens here when
                                 //I try to display all entries.
           //---------------------------------------------------
        }

        Console.WriteLine("Press any key to continue...");
        Console.ReadKey();

    }

    public static void AddPerson(ref Naam[] personen)
    {
        //----------------------------------------------------------
        StreamWriter writer = new StreamWriter("Names.txt", true);//here I get an error saying that the file is already opened
        //writer = File.AppendText("Names.txt");                  //by another process (probably the getdata function)
//--------------------------------                                               //and I don't know how to get past this

        //writer.WriteLine(personen.Length + 1);

        Naam temp = new Naam();

        //input gegevens nieuwe persoon
        Console.WriteLine("Name: ");
        temp.Name = Console.ReadLine();

        Console.WriteLine("Hometown: ");
        temp.Livingplace = Console.ReadLine();

        Console.WriteLine("Profession: ");
        temp.Profession = Console.ReadLine();

        //schrijf nieuwe gegevens naar tekst file.
        writer.WriteLine(temp.Name);
        writer.WriteLine(temp.Livingplace);
        writer.WriteLine(temp.Profession);

        //geef de vorige gegevens in.
        for (int i = 0; i < personen.Length; i++)
        {
            //-----------------------------------------
            writer.WriteLine(personen[i].Name);         //error object-reference-not-set-to-an-instance-error happens here when
                                                        //I try to add a new person even 
            writer.WriteLine(personen[i].Livingplace);
            writer.WriteLine(personen[i].Profession);
            //-----------------------------------------
        }

        writer.Close();

        //update array.
        GetData(ref personen);
    }
}

}