ScriptUI Button function is not working

Hi friends, Everything seems fine in this code, but in practice, when I select a few paragraphs of text, run the script, and click the Apply button, nothing happens.

// Create a new window
var dialog = new Window("dialog");
dialog.text = "Apply Paragraph Styles";
// Add a button to the window
var applyButton = dialog.add("button", undefined, "Apply");
// Add an event listener for the button click event
applyButton.onClick = function() {
  // Get a reference to the active document
  var doc = app.activeDocument;
  // Get a reference to the selected paragraphs
  var selParas = doc.selection[0].paragraphs;
  // Loop through the selected paragraphs and apply the paragraph styles
  for (var i = 0; i < selParas.length; i++) {
    // Apply the first paragraph style to the first paragraph
    if (i == 0) {
      selParas[i].appliedParagraphStyle = doc.paragraphStyles.itemByName("MK 1");
    }
    // Apply the second paragraph style to the last paragraph
    else if (i == selParas.length - 1) {
      selParas[i].appliedParagraphStyle = doc.paragraphStyles.itemByName("MK 3");
    }
    // Apply the first paragraph style to all paragraphs in between
    else {
      selParas[i].appliedParagraphStyle = doc.paragraphStyles.itemByName("MK 2");
    }
  }
  // Close the dialog window
  dialog.close();
}
// Show the window
dialog.show();