Create error message labels based on user input WPF C#

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C#

Hello,
I am creating WPF app in C# that creates a mathematical tasks for kids(addition, subtraction etc.). In one User Controller I have a button "Begin", which loads another UC and in that UC I have 10 tasks, beside every task I have Text Box for user to input answer and I have button "Confirm" (all of this is dynamically created), to be able to make a list of user answers when the button is clicked. But now, I have a problem with empty Text Boxes.
I want, when the button "Confirm" is clicked, to check if there are an empty Text Box-es, and if there is/are, I want Label/s to appear with some text beside that empty box/es. As soon as user start typing in that or those particular text box(es) I want that label to dissapear.

What I have tried:

This is where I create tasks and Text Boxes:

public void CreateTasksVisually(UCFirstLevelAddition userControl, List<string> additionTasksList)
        {
            int x = 350, y = 200;
            for (int i = 0; i < 10; i++)
            {
                userControl.Controls.Add(new Label
                {
                    Text = additionTasksList[i],
                    Height = 40,
                    Width = 100,
                    Name = "additionTaskLabel" + i,
                    BackColor = Color.Transparent,
                    Location = new Point(x, y + 15)
                });

                userControl.Controls.Add(new TextBox
                {
                    Name = "container" + i.ToString(),
                    Text = "",
                    Height = 20,
                    Width = 50,
                    BackColor = Color.White,
                    Location = new Point(x + 120, y + 15)

                });

                y = y + 60;
            }
        }

This is where I create a button "Confirm":

public Button CreateSubmitBtnVisually(UCFirstLevelAddition userControl)
       {
           int x = 350, y = 200;
           Button submitBtn = new Button();
           submitBtn.Name = "btnSubmit";
           submitBtn.Text = "Potvrdi";
           submitBtn.Size = new Size(100, 40);
           submitBtn.Location = new Point(350, 829);

           submitBtn.Click += new EventHandler(userControl.submitBtn_Click);

           userControl.Controls.Add(submitBtn);
           return submitBtn;
       }

And this is where I try to check if anyone of the Text Box-es is empty:

public void CreateEmptyContainerLabels(UCFirstLevelAddition userControl)
        {
            int x = 550, y = 180;
            Label message = new Label();
            foreach (Control c in userControl.Controls)
            {
                if (c is TextBox)
                {
                    TextBox container = c as TextBox;
                    if (container.Text.Length == 0)
                    {
                        message.Text = string.Format("Ne sme biti prazno!");
                        message.Width = 400;
                        message.ForeColor = Color.Red;
                        message.Font = new Font("Century Gothic", 16, FontStyle.Bold);
                        message.Location = new Point(x, y + 40);
                    }
                    userControl.Controls.Add(message);
           
                    y = y + 60;
                }
            }

        }

But this check is not doing what I want. Can you help me with this?
Thank's in advance.