Editing/Writing mpp file on c# using mpxj

I'm trying to view and edit Tasks on a mpp file using mpxj on c#. I wrote some lines and tring to write a file that is already complete adding a new Task.

But when the code arrives to the write part, it create the file in the path I meant but the file is empty and if I try to read that file it give me the System.NullReferenceException : Object reference not set to an instance of an object error. It's the first time code with this mpxj library.

 ProjectReader reader = new MPPReader();
 ProjectWriter writer = new MPXWriter();
 private ProjectFile project;

 public ProjectFile OpenReaderProject()
    {
        string filePath = "Files/Prova file1.mpp";

        try
        {
            project = reader.read(filePath);
        }
        catch (MPXJException ex)
        {
            Console.WriteLine("Errore durante l'apertura del file .mpp: " + ex.Message);
        }
        return project;
    }

    public ProjectFile OpenWriterProject()
    {
        string filePath = "Files/Prova file1.mpp";

        try
            {
            writer.write(project, filePath);
            }
            catch (MPXJException ex)
            {
                Console.WriteLine("Errore durante la scrittura del file .mpp: " + ex.Message);
            }
        return project;
    }

This is the write and reading part.

 public void AddTask()
    {
        var project = OpenReaderProject();

        var task = project.addTask();
        task.setName(java.lang.String.valueOf("Attività di esempio"));
        var taskDto = _mapper.Map<TaskDto>(task);
            OpenWriterProject();

    }`

This is the add task function and calling the OpenWriterProject.
What's wrong with it? Feel free to answer me Thanks

You will have to close the reader before using the writer. I suggest that you only create the reader and writer objects just before you use them, like this:

ProjectFile project;
string filePath = "Files/Prova file1.mpp";

public ProjectFile OpenReaderProject()
{
    ProjectReader reader = new MPPReader();
    project = reader.read(filePath);
    return project;
}

public ProjectFile OpenWriterProject()
{
    ProjectWriter writer = new MPXWriter();
    writer.write(project, filePath);
    return project;
}