Python PyQt dynamic widget generation using multithread

Question

I am writing a simple program that reads new input being written to a file and displays it in popup windows.

I am using PyQt5 for:
A) File changes (QtCore)
B) Window (QtWidgets)

Note that I do actually have a working model using signals as per PyQt reference guide. But couldn't figured out the below, therefore I am not understanding how things works.

While testing implementations i came across with multi-thread issues. This is a simplified example of my problem:

from PyQt5 import QtWidgets
import sys
import threading

def additem(window):
    """ generates label in window at bottom """
    print("Adding label to window on thread {}".format(threading.currentThread()))
    row = window.layout().rowCount() + 1
    btn = QtWidgets.QPushButton('button_' + str(row), window)
    window.layout().addWidget(btn,row,1)

if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    w = QtWidgets.QWidget()
    w.setLayout(QtWidgets.QGridLayout())
    # set to timers after 3,6 will generate label in window
    threading.Timer(3,additem,args=(w,)).start()
    threading.Timer(6,additem,args=(w,)).start()
    w.show()
    sys.exit(app.exec_())

Output:

Adding label to window
QObject::setParent: Cannot set parent, new parent is in different thread

I tried setting global queue to pass around objects but still same error.

Although I now why the exception is raised, I do not understand how to avoid it.

One attempt was to directly pass the top object "app" and then getting the window by index (app is the top parent):

...
threading.Timer(3,additem,args=(app,)).start()
...

def additem(app):
    w = app.allWidgets()[0]
    ...

I know PyQt is a C library wrapped in python but I am not familiar with C. The app object is a pointer to the object in the memory heap,
why it can't manipulate the object?
is the object being copied in a new thread? (thought I could still "update" that object)