Не работает GridLayout

import sys
from PyQt5.QtCore import QObject, QRect
from PyQt5.QtWidgets import QApplication, QWidget, QMainWindow, QAction, qApp, QPushButton, QLineEdit, QGridLayout, QDockWidget
from PyQt5.QtGui import QIcon
 
class GUI(QMainWindow):
    #Problem Widgets
    Widgets = []
 
 
    def __init__(self):
        super().__init__()
        self.initUI()
 
    def initUI(self):
        widget = QWidget(self)
        core = QObject(self)
        
        #MenuBar
        menubar = self.menuBar()
        filemenu = menubar.addMenu('&File')
 
        #grid layout
        self.ProblemLayout = QGridLayout()
        self.ProblemLayout.setGeometry(QRect(10, 40, 390, 950))
 
        #Buttons
        btnAdd = QPushButton('Add', self)
        btnAdd.resize(60, 40)
        btnAdd.move(10, 30)
        btnAdd.clicked.connect(self.AddProblem)
 
        btnADel = QPushButton('Del All', self)
        btnADel.resize(60, 40)
        btnADel.move(80, 30)
        btnADel.clicked.connect(self.DelProblem)
        
        #Window
        self.setGeometry(500, 150, 1000, 800)
        self.setWindowTitle('Planner')
        self.setWindowIcon(QIcon('images/icon/icon.png'))
 
        self.show()
 
    def AddProblem(self):
        
        self.lineEditP = QLineEdit(self)
 
        self.btnP = QPushButton('More', self)
 
        GUI.Widgets.append([self.lineEditP, self.btnP])
 
        for i in range(len(GUI.Widgets[-1])):
            self.ProblemLayout.addWidget(GUI.Widgets[-1][i], i, len(GUI.Widgets[-1])-1)
            GUI.Widgets[-1][i].show()
 
    #def DelProblem(self):
        #GUI.Widgets[-1][0].deleteLater()
        #GUI.Widgets[-1][1].deleteLater()
        #del GUI.Widgets[-1]
 
    #def UpdateProblem(self):
 
if __name__ == '__main__':
    app = QApplication(sys.argv)
    gui = GUI()
    sys.exit(app.exec_())