Значение не может быть неопределенным при заполнении ProxyListBox

struct ProxyLoaderParams
    {
        public Log log;
        public String FileName;
 
        public static ProxyLoaderParams BuildStructure(Log log_ex, String FileName_ex)
        {
            ProxyLoaderParams param = new ProxyLoaderParams();
            param.log = log_ex;
            param.FileName = FileName_ex;
            return param;
        }
    }
 
    class ProxyLoader
    {
        public String[] ProxyArray
        {
            get;
            private set;
        }
 
        private void LoadProxy(Object parameter)
        {
            ProxyLoaderParams param = (ProxyLoaderParams)parameter;
            try { this.ProxyArray = File.ReadAllLines(param.FileName); }
            catch (ArgumentNullException) { param.log.AddErrorMessage("Failed to load proxy list. Argument is null!"); }
            catch (ArgumentException) { param.log.AddErrorMessage("Failed to load proxy list.Invalid argument!"); }
            catch (PathTooLongException) { param.log.AddErrorMessage("Failed to load proxy list. Invalid filename length!"); }
            catch (DirectoryNotFoundException) { param.log.AddErrorMessage("Failed to load proxy list. Dir not found!"); }
            catch (FileNotFoundException) { param.log.AddErrorMessage("Failed to load proxy list. File not found!"); }
            catch (UnauthorizedAccessException) { param.log.AddErrorMessage("Failed to load proxy list. Access denied!"); }
            catch (IOException) { param.log.AddErrorMessage(@"Failed to load proxy list. Input\Output error!"); }            
 
        }
 
        public ProxyLoader(Log log,String FileName)
        {
            new Thread(LoadProxy).Start(ProxyLoaderParams.BuildStructure(log, FileName));
        }
    }

        private void LoadProxyFromFileButton_Click(object sender, EventArgs e)
        {
            ProxyLoader px_loader = new ProxyLoader(log,ProxySourcePath.Text);
            if(ProxyListBox.Items.Count != 0)    
                ProxyListBox.Items.Clear();
 
            ProxyListBox.Items.AddRange(px_loader.ProxyArray);
        }