Why is window not visible in omniverse ui kit?

Question

I am trying to show a omniverse ui kit window from a method of class, it is not working, but it is showing when i use same statement outside the method and inside that class.

Code is below.

py_ext.py

import NodeStarter    
class WirelessAppUtilities(omni.ext.IExt):
     def __init__(self):
          super().__init__()
          self._console_window = None
          self._hide_render_settings_task = None
    
     def on_startup(self, ext_id):
          self.__disable_context_menu()
          self.__set_selection_mode()
          self.__hide_main_menu()
          self.__hide_windows()
    
          NodeStarter.start_node()

    def on_shutdown(self):
        self._menu_bar = None
        self._context_menu = None

    def __hide_windows(self):
        hidden_windows = ["main toolbar", "timeline toolbar", "render settings"]
        ......

node_start.py(Not working)

import omni.ui as ui

class StartUpImage:
    def __init__(self, title: str) -> None:
        self._panel_window = None
        self._title = title

    def show_window(self):
        if self._panel_window is None:
            self._panel_window = ui.Window(self._title, width=600, height=400)
        self._panel_window.visible = True

    def close_window(self):
        if self._panel_window is not None:
            self._panel_window.visible = False

class NodeStarter(object):
    def start_node():
        sui = StartUpImage(title="Starting App")
        sui.show_window() **----> Not showing window here**
        pass

node_start.py(Working)

import omni.ui as ui

class StartUpImage:
    def __init__(self, title: str) -> None:
        self._panel_window = None
        self._title = title

    def show_window(self):
        if self._panel_window is None:
            self._panel_window = ui.Window(self._title, width=600, height=400)
        self._panel_window.visible = True

    def close_window(self):
        if self._panel_window is not None:
            self._panel_window.visible = False

class NodeStarter(object):
    sui = StartUpImage(title="Starting App")
    sui.show_window() **----> showing window here**
    def start_node():
        pass

I want to show window from method of class, how can i achieve this?