运行后提示这个错误，看了半天都不知道哪边出了问题。求高手指点，感激不敬

FileSystemInfo[] FSInfo;
string Fpath;
string path;
int i=0;

private void GetPic()
        {
            if(i<FSInfo.Length)
            {
                string FileType=FSInfo[i].ToString().Substring(FSInfo[i].ToString().LastIndexOf(".")+1);
                FileType=FileType.ToLower();
                if(FileType=="bmp"||FileType=="gif"||FileType=="jpeg"||FileType=="png")
                    pictureBox1.Image=Image.FromFile(Fpath+FSInfo[i].ToString());
            }
        }

        private void menuItem2_Click(object sender, System.EventArgs e)
        {
            if(folderBrowserDialog1.ShowDialog()==DialogResult.OK)
            {
                path=folderBrowserDialog1.SelectedPath;
                DirectoryInfo dir=new DirectoryInfo(path);
                FSInfo=dir.GetFileSystemInfos();
                if(path.Length<=4)
                    Fpath=path;
                else
                    Fpath=path+"\\";
            }
        }

        private void timer1_Tick(object sender, System.EventArgs e)
        {
            GetPic();
            if(i==FSInfo.Length-1)
                i=0;
            else
                i++;
        }

谢谢！