VTK Plot Window Freezes Main GUI

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I have a program that is written in Python. GUI uses GTK. I plot something by using VTK and it works. But main GUI freezes and other buttons, etc do not work. I have tried multithreading but it still freezes. Maybe Python’s main loop and/or VTK’s loop have problems with working other.

EDIT: When program encounters an error at the end of the plotting codes (before iren.Start()), both plot window and main GUI works. (For example, when x=y code is placed without defining x and y it gives error.) But there is an error. There may be another solution for that.

What is the solution?

Python 3.8
GTK 3
VTK 8.2
Windows 10 (x64)

Here is the code:

import gi
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk
import vtk

builder = Gtk.Builder()
builder.add_from_file('test.glade')
builder.get_objects()
window1 = builder.get_object('window1')

class Signals:
    def on_window1_destroy(self, widget):
        Gtk.main_quit()

    def on_button1_clicked(self, widget):
        cylinder = vtk.vtkCylinderSource()
        cylinderMapper = vtk.vtkPolyDataMapper()
        cylinderMapper.SetInputConnection(cylinder.GetOutputPort())
        cylinderActor = vtk.vtkActor()
        cylinderActor.SetMapper(cylinderMapper)

        ren = vtk.vtkRenderer()
        renWin = vtk.vtkRenderWindow()
        renWin.AddRenderer(ren)
        iren = vtk.vtkRenderWindowInteractor()
        iren.SetRenderWindow(renWin)

        ren.AddActor(cylinderActor)
        renWin.Render()

        iren.Initialize()
        iren.Start()

builder.connect_signals(Signals())
window1.show_all()
Gtk.main()