Process.Start returns null

I'm writing a program that launches a random file in s directory. the file can be of any type, but mostly video or image files. Each time I launch a file I want to close the previous opened one.

Code :

string FolderSelected = "";
string FileName = "";
Process proc;
            
List<string> FilesDisplayed = new List<string>();

private void button2_Click(object sender, EventArgs e)
{
    if (FolderSelected == string.Empty)
        FolderSelected = Properties.Settings.Default.FilesDefaultFolder;

    if (proc != null)
    {
        proc.CloseMainWindow();
        proc.Close();
    }
    FileName = FetchRandomFile();
    proc = Process.Start(FileName);
}

Problem is, that I keep getting proc = null (the file is launched properly) and I cannot fetch the previously opened process in order to close it. I know that .NET reuses processes and that's why it returns Null but I need to override this behavior.