private void guna2Button8_Click(object sender, EventArgs e)
{
    // Create the UC_ListBoxhub user control and pass the Form1 reference to it
    UC_ListBoxhub uc = new UC_ListBoxhub(this); // Passing reference to Form1
    addUserControl(uc); // Add the user control
}

 private void addUserControl(UserControl userControl)
 {
     // Loop through controls and hide any other user control (but keep FastColoredTextBox visible)
     foreach (Control ctrl in this.Controls)
     {
         if (ctrl is UserControl && ctrl != userControl) // Hide other user controls
         {
             ctrl.Hide();
         }
     }

     // If the user control is not already added, add it
     if (!this.Controls.Contains(userControl))
     {
         userControl.Dock = DockStyle.Fill; // Make sure it fills the form
         Controls.Add(userControl); // Add the user control
     }

     userControl.BringToFront(); // Bring the new user control to the front
 }

private void guna2Button8_Click(object sender, EventArgs e)
{
    // Create the UC_ListBoxhub user control and pass the Form1 reference to it
    UC_ListBoxhub uc = new UC_ListBoxhub(this); // Passing reference to Form1
    addUserControl(uc); // Add the user control
}

 private void addUserControl(UserControl userControl)
 {
     // Loop through controls and hide any other user control (but keep FastColoredTextBox visible)
     foreach (Control ctrl in this.Controls)
     {
         if (ctrl is UserControl && ctrl != userControl) // Hide other user controls
         {
             ctrl.Hide();
         }
     }

     // If the user control is not already added, add it
     if (!this.Controls.Contains(userControl))
     {
         userControl.Dock = DockStyle.Fill; // Make sure it fills the form
         Controls.Add(userControl); // Add the user control
     }

     userControl.BringToFront(); // Bring the new user control to the front
 }