std::vector<QWidget*> daytask_widget_vec;
QWidget* daytask_widget;

auto index = std::find(daytask_widget_vec.begin(), daytask_widget_vec.end(), daytask_widget);
if (index != daytask_widget_vec.end()) {
    QWidget* tmp_widget = *index;
    index = daytask_widget_vec.erase(index);
    delete tmp_widget;
}

//ошибочка