PyQt5 code not working

[tchachkii]

so I'm new to PyQt5 and have am trying to open a new window when a button is pressed. I know this should be simple and have managed to copy other examples to produce my code but it doesn't run - the second window opens then instantly closes. this should be a simple fix but I don't get where I've gone wrong, could anyone help me? thanks

from PyQt5.QtWidgets import *
 
class StartScreen(QWidget):
    def __init__(self):
        self.window = QWidget()
        self.window.setGeometry(50, 50, 350, 200)
        self.window.setWindowTitle("Fantasy F1")
        self.UserOrNot()
        self.window.show()
 
    def UserOrNot(self):
        self.layout = QGridLayout()
        self.UserQuestion = QLabel("Are you an existing user or a new user?")
        self.layout.addWidget(self.UserQuestion)
 
        ## creates a button for existing users
        self.ExistingUser = QPushButton("Existing user")
        self.layout.addWidget(self.ExistingUser)
 
        ## creates a button for new users
        self.NewUser = QPushButton("New user")
        self.NewUser.clicked.connect(NewUserPressed)
        self.layout.addWidget(self.NewUser)
 
        self.window.setLayout(self.layout)
       # self.window.show()
 
 
def NewUserPressed():
    print("button pressed")
    main = CreateUser()        
 
class CreateUser(QWidget):
    def __init__(self):
        self.window = QWidget()
        self.window.setGeometry(50, 50, 350, 250)
        self.window.setWindowTitle("Create a team")
        self.CreateUserForm()
 
 
    def CreateUserForm(self):
        self.layout = QFormLayout()
 
        ## creates a form (age is spinbox to make sure user can only enter numbers)
        self.layout.addRow(QLabel("*Username:"), QLineEdit())
        self.layout.addRow(QLabel("*Team name:"), QLineEdit())
        self.layout.addRow(QLabel("*First name:"), QLineEdit())
        self.layout.addRow(QLabel("*Last name:"), QLineEdit())
        self.layout.addRow(QLabel("*Password:"), QLineEdit())
        self.layout.addRow(QLabel("Email address:"), QLineEdit())
        self.layout.addRow(QLabel("Age:"), QSpinBox())
        self.layout.addRow(QLabel("Those with a * must be filled in."))
 
        self.OKButton = QPushButton("OK")
        self.layout.addWidget(self.OKButton)
 
        self.CancelButton = QPushButton("Cancel")
        self.layout.addWidget(self.CancelButton)
 
        self.window.setLayout(self.layout)
        self.window.show()
 
 
if __name__ == '__main__':
    import sys
 
    app = QApplication(sys.argv)
    main = StartScreen()
 
 
    sys.exit(app.exec_())

from PyQt5.QtWidgets import *

class StartScreen(QWidget):
    def __init__(self):
        self.window = QWidget()
        self.window.setGeometry(50, 50, 350, 200)
        self.window.setWindowTitle("Fantasy F1")
        self.UserOrNot()
        self.window.show()
        
    def UserOrNot(self):
        self.layout = QGridLayout()
        self.UserQuestion = QLabel("Are you an existing user or a new user?")
        self.layout.addWidget(self.UserQuestion)

        ## creates a button for existing users
        self.ExistingUser = QPushButton("Existing user")
        self.layout.addWidget(self.ExistingUser)
        
        ## creates a button for new users
        self.NewUser = QPushButton("New user")
        self.NewUser.clicked.connect(NewUserPressed)
        self.layout.addWidget(self.NewUser)
        
        self.window.setLayout(self.layout)
       # self.window.show()
 

def NewUserPressed():
    print("button pressed")
    main = CreateUser()        

class CreateUser(QWidget):
    def __init__(self):
        self.window = QWidget()
        self.window.setGeometry(50, 50, 350, 250)
        self.window.setWindowTitle("Create a team")
        self.CreateUserForm()
        

    def CreateUserForm(self):
        self.layout = QFormLayout()

        ## creates a form (age is spinbox to make sure user can only enter numbers)
        self.layout.addRow(QLabel("*Username:"), QLineEdit())
        self.layout.addRow(QLabel("*Team name:"), QLineEdit())
        self.layout.addRow(QLabel("*First name:"), QLineEdit())
        self.layout.addRow(QLabel("*Last name:"), QLineEdit())
        self.layout.addRow(QLabel("*Password:"), QLineEdit())
        self.layout.addRow(QLabel("Email address:"), QLineEdit())
        self.layout.addRow(QLabel("Age:"), QSpinBox())
        self.layout.addRow(QLabel("Those with a * must be filled in."))

        self.OKButton = QPushButton("OK")
        self.layout.addWidget(self.OKButton)
               
        self.CancelButton = QPushButton("Cancel")
        self.layout.addWidget(self.CancelButton)
        
        self.window.setLayout(self.layout)
        self.window.show()
        

if __name__ == '__main__':
    import sys
    
    app = QApplication(sys.argv)
    main = StartScreen()
    

    sys.exit(app.exec_())

To copy to clipboard, switch view to plain text mode