Can't embed boxes inside of other boxes

[MelihDarcanxyz]

I'm having difficulties embedding boxes inside other boxes. The window renders fine at first, but crashes the moment an update happens. MilestoneBox itself is a subclass of widgets.Box. Here's the code snippet:

        super().__init__(
            vertical=True, # <---- Vertical
            spacing=10,
            child=self.milestone_box.bind("value", lambda value: [            # <---- I want these to be non-vertical
                    widgets.CenterBox(
                        start_widget=self.prev_button,
                        center_widget=widgets.Label(label=self.data.name),
                        end_widget=self.next_button
                    ),
                    value,
                ]
            )
        )

    # Update functions
    def view_prev(self, button_obj):
        if self.cursor_ > 0:
            self.cursor_ -= 1

        self.milestone_box.value = self.milestone_boxes[self.cursor_] # <---- .value attributes are updated

    def view_next(self, button_obj):
        if self.cursor_ < (len(self.milestone_boxes) - 1):
            self.cursor_ += 1

        self.milestone_box.value = self.milestone_boxes[self.cursor_] # <---- .value attributes are updated

I keep getting this error, whatever I do. Got error from Box, CenterBox, and Grid.

2025-07-31 01:54:52 [CRITICAL] (python3.13:148703): Gtk-CRITICAL **: 01:54:52.661: gtk_center_box_set_start_widget: assertion 'child == NULL || self->start_widget == child || gtk_widget_get_parent (child) == NULL' failed
2025-07-31 01:54:52 [CRITICAL] (python3.13:148703): Gtk-CRITICAL **: 01:54:52.661: gtk_center_box_set_end_widget: assertion 'child == NULL || self->end_widget == child || gtk_widget_get_parent (child) == NULL' failed

Again, the layout is fine at first, the update causes this problem. I first thought that we're not able to nest boxes inside each other, that it's how gtk was implemented, but then saw there were many layers of boxes nested in example configurations. There must be something I'm doing wrong. Thanks in advance.

[lost-melody]

self.milestone_box.bind("value", lambda value: [...]) creates a new CenterBox every time value changes, while self.prev_button and self.next_button are children of the previous CenterBox and cannot be set as children of the new one (where assertion gtk_widget_get_parent (child) == NULL failed).

[MelihDarcanxyz]

Thanks for the reply. Okay, that's a lot more clear now. What would be the correct way of doing what I'm trying to do?

[lost-melody]

Try this:

super().init(
  child=[
    widgets.CenterBox(**kwargs),
    widgets.Box(
      child=self.milestone_boxes.bind("value", lambda value: [value]),
    ),
  ],
)

[MelihDarcanxyz]

Worked like a charm. Thanks. So the idea was that isolate the Binding as much as possible.