PyQ5中findChild() 方法，为什么是none

第一个代码是kimi生成的，调用findchild方法可以使用；第二个调用findchild是none，为什么？

import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QLineEdit

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setGeometry(100, 100, 300, 200)
        self.setWindowTitle('Find Child Example')

        # 创建一个 QLineEdit 控件并设置对象名称
        lineEdit = QLineEdit(self)
        lineEdit.setObjectName("lineEdit")
        lineEdit.setGeometry(50, 50, 200, 30)

        # 使用 findChild 查找控件
        foundLineEdit = self.findChild(QLineEdit, "lineEdit")
        if foundLineEdit:
            print("找到 QLineEdit 控件:", foundLineEdit)
        else:
            print("未找到 QLineEdit 控件")

if __name__ == '__main__':
    app = QApplication(sys.argv)
    mainWindow = MainWindow()
    mainWindow.show()
    sys.exit(app.exec_())

使用qtdesigner生成，没有使用ui和代码分离。findchild为什么是none？

cheng2.py

# -*- coding: utf-8 -*-

# Form implementation generated from reading ui file 'cheng2.ui'
#
# Created by: PyQt5 UI code generator 5.15.9
#
# WARNING: Any manual changes made to this file will be lost when pyuic5 is
# run again.  Do not edit this file unless you know what you are doing.


from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtCore import QObject
from PyQt5.QtWidgets import QLineEdit


class Ui_MainWindow(QObject):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        MainWindow.setStyleSheet("font: 75 14pt \"Adobe Arabic\";")
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.widget = QtWidgets.QWidget(self.centralwidget)
        self.widget.setGeometry(QtCore.QRect(220, 60, 261, 351))
        self.widget.setObjectName("widget")
        self.verticalLayout = QtWidgets.QVBoxLayout(self.widget)
        self.verticalLayout.setContentsMargins(0, 0, 0, 0)
        self.verticalLayout.setObjectName("verticalLayout")
        self.lineEdit = QtWidgets.QLineEdit(self.widget)
        self.lineEdit.setObjectName("lineEdit")
        self.verticalLayout.addWidget(self.lineEdit)
        self.lineEdit_2 = QtWidgets.QLineEdit(self.widget)
        self.lineEdit_2.setObjectName("lineEdit_2")
        self.verticalLayout.addWidget(self.lineEdit_2)
        self.lineEdit_3 = QtWidgets.QLineEdit(self.widget)
        self.lineEdit_3.setObjectName("lineEdit_3")
        self.verticalLayout.addWidget(self.lineEdit_3)
        self.lineEdit_4 = QtWidgets.QLineEdit(self.widget)
        self.lineEdit_4.setObjectName("lineEdit_4")
        self.verticalLayout.addWidget(self.lineEdit_4)
        self.lineEdit_5 = QtWidgets.QLineEdit(self.widget)
        self.lineEdit_5.setObjectName("lineEdit_5")
        self.verticalLayout.addWidget(self.lineEdit_5)
        self.lineEdit_6 = QtWidgets.QLineEdit(self.widget)
        self.lineEdit_6.setObjectName("lineEdit_6"s)
        self.verticalLayout.addWidget(self.lineEdit_6)
        self.pushButton = QtWidgets.QPushButton(self.widget)
        self.pushButton.setObjectName("pushButton")
        self.verticalLayout.addWidget(self.pushButton)
        self.lineEdit_7 = QtWidgets.QLineEdit(self.widget)
        self.lineEdit_7.setObjectName("lineEdit_7")
        self.verticalLayout.addWidget(self.lineEdit_7)
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 800, 28))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

        num1 = self.findChild(QLineEdit, "lineEdit")
        print(num1)


        self.pushButton.clicked.connect(self.multiplication)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton.setText(_translate("MainWindow", "乘2"))

    def multiplication(self):
        print("kk")
        num1 = self.findChild(QLineEdit, "lineEdit")
        num1.setText(num1.text() * 2)
        print(num1)
        num2 = self.findChild(QLineEdit, "lineEdit_2")
        num3 = self.findChild(QLineEdit, "lineEdit_3")
        num4 = self.findChild(QLineEdit, "lineEdit_4")
        num5 = self.findChild(QLineEdit, "lineEdit_5")
        num6 = self.findChild(QLineEdit, "lineEdit_6")
        num7 = self.findChild(QLineEdit, "lineEdit_7")

        for i in range(1,7):
            num = f'num{i}'
            print(num)
            num.setText(num.text() * 2)



import sys
# 主方法，程序从此处启动PyQt设计的窗体
if __name__ == '__main__':
   app = QtWidgets.QApplication(sys.argv)
   MainWindow = QtWidgets.QMainWindow() # 创建窗体对象
   ui = Ui_MainWindow() # 创建PyQt设计的窗体对象
   ui.setupUi(MainWindow) # 调用PyQt窗体的方法对窗体对象进行初始化设置
   MainWindow.setWindowFlags(QtCore.Qt.Window)
   MainWindow.show() # 显示窗体
   sys.exit(app.exec_()) # 程序关闭时退出进程