### Godot version

4.1.dev (dc625bcbfc5b587ba8867481ee7a1f6a567c25d0)

### System information

Mageia 9, Linux

### Issue description

When importing a project in current `master`, the following error is raised for each asset that gets imported:
```
Attempting to parent and popup a dialog that already has a parent.
```

It seems to be raised once per asset, caused by the progress dialog.

CC @bruvzg

### Steps to reproduce

- Import a new project in Godot that contains importable assets, e.g. https://github.com/RPicster/godot4-demo-desert-light

### Minimal reproduction project

See above.
if (MessageQueue::get_singleton()->is_flushing()) {
		ERR_PRINT("Do not use progress dialog (task) while flushing the message queue or using call_deferred()!");
		return;
	}
	ERR_FAIL_COND_MSG(tasks.has(p_task), "Task '" + p_task + "' already exists.");
	ProgressDialog::Task t;
	t.vb = memnew(VBoxContainer);
	VBoxContainer *vb2 = memnew(VBoxContainer);
	t.vb->add_margin_child(p_label, vb2);
	t.progress = memnew(ProgressBar);
	t.progress->set_max(p_steps);
	t.progress->set_value(p_steps);
	vb2->add_child(t.progress);
	t.state = memnew(Label);
	t.state->set_clip_text(true);
	vb2->add_child(t.state);
	main->add_child(t.vb);
	tasks[p_task] = t;
	if (p_can_cancel) {
		cancel_hb->show();
	} else {
		cancel_hb->hide();
	}