old_contents: import extractor.ui.mainapplication as ui
  import Tkinter as tk
  
  def main():
      root = tk.Tk()
      root.title('SNES Wolfenstein 3D Extractor')
      root.minsize(400, 100)
      ui.MainApplication(root).pack(side="top", fill="both", expand=True)
      root.mainloop()
  
  main()
  
new_contents: import extractor.ui.mainapplication as ui
  import Tkinter as tk
  
  def main():
  ##    root = tk.Tk()
  ##    root.title('SNES Wolfenstein 3D Extractor')
  ##    root.minsize(400, 100)
  ##    ui.MainApplication(root).pack(side="top", fill="both", expand=True)
  ##    root.mainloop()
      ui.MainApplication().mainloop()
  
  main()
  
subject: Use new MainApplication Tk class.