Why widgets are not displayed in PyQt6?

Question

I am implementing an app through a class that has a QMainWindow member. I call two functions (create_side_menu, create_mixing_window) that display dynamically added widgets inside a bigger widget, which is added to the QMainWindow. The first function works, while the second does not, even though very similar to the first. Can you help me solving this problem?

Update: I modified the code as it appears now in the question, and debugging it seems that the problem is at line 78 (wid = ChannelModifier(af)) where it throws the error: "QWidget: Must construct a QApplication before a QWidget" even though the application is defined in MainWindow class init

Notes:

• ChannelModifier is a custom class deriving from QWidget

    class MainWindow:
        def __init__(self):
            self.QListWidgetLeft = None  # QListWidget
            self.QListWidgetRight = None  # QListWidget
            self.central_widget = None
            self.manager = Manager.get_instance()
            self.app = QApplication(sys.argv)
            self.window = QMainWindow()
            self.set_window()
            self.set_menu_bar()
            self.sb = self.window.statusBar()
            self.sb.showMessage(f"ready to remix")
            self.window.show()
            sys.exit(self.app.exec())
  
        def set_window(self):
            # self.window.setFixedSize(1080, 720)
            self.window.setGeometry(50, 50, 1500, 1000)
            self.window.setWindowIcon(QIcon("assets/icon.png"))
            # self.window.setStyleSheet('background-color: #b7dfeb')
            self.window.setStyleSheet('background-color:qlineargradient(spread:pad, x1:0, y1:0, x2:1, y2:1, stop:0 '
                                      'rgba(165, 165, 247, 255), stop:1 rgba(255, 255, 255, 255))')
            self.window.setWindowTitle("Music Remixes and Mashups")
            self.create_central_widget()
  
        def create_central_widget(self):
            self.central_widget = QWidget()
            self.window.setCentralWidget(self.central_widget)
            self.central_widget.setLayout(QHBoxLayout())
            self.create_side_menu()
            self.create_mixing_window()
  
        def create_side_menu(self):
            widget = QWidget()
            vbox = QVBoxLayout()
            scrollbar = QScrollBar()
            scrollbar.setMaximum(100)
            scrollbar.setStyleSheet("background-color: rgb(60,60,90); width: 14px; border-radius 0px;")
            scrollbar.sliderMoved.connect(scrollbar.value)
            self.QListWidgetLeft = QListWidget()
            self.QListWidgetLeft.setSpacing(5)
            self.QListWidgetLeft.setVerticalScrollBar(scrollbar)
            self.QListWidgetLeft.setVerticalScrollBarPolicy(Qt.ScrollBarPolicy.ScrollBarAlwaysOn)
  
            pr = self.manager.get_current_project()
            if pr:
                for af in pr.get_audio_files():
                    self.add_audiofile_to_side_menu(af)
            vbox.addWidget(self.QListWidgetLeft)
            widget.setLayout(vbox)
            widget.setFixedWidth(230)
            widget.setStyleSheet('background-color:white')
            self.central_widget.layout().addWidget(widget)
  
        def add_audiofile_to_side_menu(self, af: AudioFile):
            if self.QListWidgetLeft is None:
                self.QListWidgetLeft = QListWidget()
            label = QLabel()
            label.setFixedWidth(180)
            label.setFixedHeight(180)
            if af.get_thumb_path():
                label.setPixmap(QPixmap(af.get_thumb_path()))
            else:
                label.setText(af.get_title())
            item = QListWidgetItem(self.QListWidgetLeft)
            item.setSizeHint(label.size())
            self.QListWidgetLeft.addItem(item)
            self.QListWidgetLeft.setItemWidget(item, label)
  
        def create_mixing_window(self):  # TODO: it doesn't work
            mixing_window = QWidget()
            grid = QGridLayout()
  
            pr = self.manager.get_current_project()
            if pr:
                for af in pr.get_audio_files():
                    # self.add_channel_modifier_to_mixing_menu(af)
                    wid = ChannelModifier(af)
                    grid.addWidget(wid)
            # grid.addWidget(self.QListWidgetRight)
            mixing_window.setLayout(grid)
            mixing_window.setStyleSheet('background-color:white')
            self.central_widget.layout().addWidget(mixing_window)
  
        def add_channel_modifier_to_mixing_menu(self, af: AudioFile):
            if self.QListWidgetRight is None:
                self.QListWidgetRight = QListWidget()
            wid = ChannelModifier(af)
            item = QListWidgetItem(self.QListWidgetRight)
            # item.setSizeHint(wid.size())
            self.QListWidgetRight.addItem(item)
            self.QListWidgetRight.setItemWidget(item, wid)
  

Answer 1

With the limited amount of code you have included it is difficult to say if this is what is causing your problem or not, but one issue I see is that you aren't using the correct signature in your calls to addWidget() on your grid layout.

With grid layout you have to specify the column and row you want the widget to occupy when adding them to the layout.

def create_mixing_window(self):  # TODO: it doesn't work
    mixing_window = QWidget()
    grid = QGridLayout()
    pr = self.manager.get_current_project()
    if pr:
        for af in pr.get_audio_files():
            self.add_channel_modifier_to_mixing_menu(af)
            wid = ChannelModifier(af)
            grid.addWidget(wid,0,0)  # row 0; column 0
            grid.addWidget(self.QListWidgetRight, 0, 1)  # row 0; column 1
    mixing_window.setLayout(grid)
    mixing_window.setStyleSheet('background-color:white')
    self.central_widget.layout().addWidget(mixing_window)