Cannot save a new file after saving previous deleted file C#

Question

So I have three buttons that have its own functionalities,

Generate a TextBox dynamically

Retrieve texts from the textbox and store them in a text file

Delete the text files.

Generate textbox button:

// Generate notes
        int curr = 0;
        private void guna2Button1_Click(object sender, EventArgs e) {
            int top = 25;
            int h_p = 170;
            curr++;
                var new_note = new Guna2TextBox() {
                    Text = "Title\n",
                    Name = "Note" + curr,
                    Multiline = true,
                    AcceptsTab = true,
                    AcceptsReturn = true,
                    WordWrap = false,
                    ScrollBars = ScrollBars.Vertical,
                    Width = 220,
                    Height = 110,
                    BorderRadius = 8,
                    Font = new Font("Bahnschrift", 13),
                    ForeColor = Color.White,
                    FillColor = ColorTranslator.FromHtml("#1E1E1E"),
                    BorderColor = ColorTranslator.FromHtml("#2C2C2C"),
                    Location = new Point(450,top)
                };
            top += h_p;
            flowLayoutPanel1.Controls.Add(new_note);
        }

Stores TextBox texts into a text file button:

    // Save notes
    private void guna2Button7_Click(object sender, EventArgs e) {
        for(int j=1; j<flowLayoutPanel1.Controls.Count+1; j++) {
            var notes = ((Guna2TextBox)flowLayoutPanel1.Controls["Note" + j]);         
            var messages = notes.Text;
            var titles = notes.Lines[0];
            string notes_path = @"C:\NOTES_LOC\DATA_LOC\" + titles + ".txt"; 
            try {
                if(titles != "") { 
                    using(FileStream fs = File.Create(notes_path)) {
                        Byte[] notes_texts = new UTF8Encoding(true).GetBytes(messages);
                        fs.Write(notes_texts,0,notes_texts.Length);
                     }
               } else {
                   MessageBox.Show("Title cannot be empty", "Flow Notes System");
                }
            } catch (Exception eq) {
                MessageBox.Show(eq.ToString(),"Notes System");
            }
        }
    }

Delete stored text file button:

// Delete notes
    private void guna2Button5_Click(object sender, EventArgs e) {
        DirectoryInfo dirs_notes_source = new DirectoryInfo(@"C:\NOTES_LOC\DATA_LOC\");
        string dirs_notes = @"C:\NOTES_LOC\DATA_LOC\";
        string garb_path = @"C:\NOTES_LOC\GARBAGE_LOC\";
        if(flowLayoutPanel1.Controls.Count != 0) {
            flowLayoutPanel1.Controls.Clear();

            foreach(string dir_path in Directory.GetDirectories(dirs_notes,"*",SearchOption.AllDirectories)) {
                Directory.CreateDirectory(dir_path.Replace(dirs_notes,garb_path));
            }

            foreach(string retv_notes in Directory.GetFiles(dirs_notes,"*.*",SearchOption.AllDirectories)) {
                File.Copy(retv_notes,retv_notes.Replace(dirs_notes,garb_path),true);
            }

            foreach (FileInfo notes in dirs_notes_source.GetFiles()) {
                notes.Delete();
            }
        }
        else {
            MessageBox.Show("No note found","Flow Notes System");
        }
    }

Now let me explain my problem, when I generated the TextBox by pressing the button that supposed to generates it I tested out my "Save" which supposed to retrieve the text from the TextBox and store it in a txt file button and it works, then I tried to tested out my "Delete Notes" button and it works, but the real problem comes in when I tried to save a newly generated Note (TextBox) to a txt file after deleting the previously created note where I got this error popped up saying: Object reference not set to an instance of an object.

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
   at FlowNotes.Form1.guna2Button7_Click(Object sender, EventArgs e) in C:\Users\USER\source\repos\FlowNotes\FlowNotes\Form1.cs:line 136
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at Guna.UI2.WinForms.Guna2Button.OnClick(EventArgs e)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

On debugging process:

var messages = notes.Text;

System.NullReferenceException: 'Object reference not set to an instance of an object.'
notes was null.

Even though when the var messages = notes.Text isn't null the error above keeps showing.