qt 5.8 -> 5.9 logic_error - c++

Question

Recently updated from qt 5.8 to qt 5.9 and I'm getting some problems.
Pressing on a button that is not connected to any slot gives me:
terminate called after throwing an instance of 'std::logic_error'
what(): basic_string::_M_construct null not valid
Which is quite odd because before the update, it worked flawlessly. I'll paste some parts of the code so it will be a bit more clear.
First of all GUI is a class that inherits from QWidget.
#ifndef GUI_H
#define GUI_H
#include <QWidget>
#include <QHBoxLayout>
#include <QPushButton>
#include <QVBoxLayout>
#include <QLabel>
#include <QLineEdit>
#include <QListWidget>
#include "Controller.h"
class GUI : public QWidget
{
Q_OBJECT
public:
GUI(Controller* ctrl);
void start();
~GUI();
private:
QLineEdit* txt_title;
QLineEdit* txt_desc;
QLineEdit* txt_type;
QLineEdit* txt_dur;
QPushButton* add_btn;
QPushButton* upd_btn;
QPushButton* rm_btn;
QPushButton* fil_desc_btn;
QPushButton* fil_type_btn;
QPushButton* fil_search_btn;
QPushButton* sort_title_btn;
QPushButton* sort_desc_btn;
QPushButton* sort_type_btn;
QPushButton* undo_btn;
QPushButton* refresh_btn;
QPushButton* shuffle_btn;
QListWidget* activities;
Controller* ctrl;
void setup_gui();
void connect_gui();
void add_to_list(Activity&);
void update_list();
public slots:
void add();
void edit();
void remove();
void search();
void undo();
void refresh_list();
void sort_title();
void sort_desc();
void sort_type();
void filter_desc();
void filter_type();
void shuffle();
};
#endif
And the methods that are relevant:
GUI::GUI(Controller* _ctrl)
{
this->ctrl = _ctrl;
this->setup_gui();
this->connect_gui();
setAttribute(Qt::WA_DeleteOnClose);
}
void GUI::add_to_list(Activity& activity)
{
string str = "";
str += activity.get_title() + "; " + activity.get_description() + "; " + activity.get_type() + "; " + std::to_string(activity.get_duration());
new QListWidgetItem(QString::fromStdString(str), this->activities);
}
void GUI::setup_gui()
{
this->setContentsMargins(10, 10, 10, 10);
QHBoxLayout* main_lay = new QHBoxLayout(this);
QVBoxLayout* left_lay = new QVBoxLayout();
QVBoxLayout* right_lay = new QVBoxLayout();
QHBoxLayout* title_lay = new QHBoxLayout();
QLabel* lbl_title = new QLabel(QObject::tr("title"));
lbl_title->setAlignment(Qt::AlignCenter);
this->txt_title = new QLineEdit();
QHBoxLayout* desc_lay = new QHBoxLayout();
QLabel* lbl_desc = new QLabel(QObject::tr("description"));
lbl_desc->setAlignment(Qt::AlignCenter);
this->txt_desc = new QLineEdit();
QHBoxLayout* type_lay = new QHBoxLayout();
QLabel* lbl_type = new QLabel(QObject::tr("type"));
lbl_type->setAlignment(Qt::AlignCenter);
this->txt_type = new QLineEdit();
QHBoxLayout* dur_lay = new QHBoxLayout();
QLabel* lbl_dur = new QLabel(QObject::tr("duration"));
lbl_dur->setAlignment(Qt::AlignCenter);
this->txt_dur = new QLineEdit();
QHBoxLayout* op_lay = new QHBoxLayout();
this->add_btn = new QPushButton(QObject::tr("&Add"));
this->upd_btn = new QPushButton(QObject::tr("Updat&e"));
this->rm_btn = new QPushButton(QObject::tr("&Remove"));
QHBoxLayout* fil_lay = new QHBoxLayout();
this->fil_desc_btn = new QPushButton(QObject::tr("FilterBy&Desc"));
this->fil_type_btn = new QPushButton(QObject::tr("FilterBy&Type"));
this->fil_search_btn = new QPushButton(QObject::tr("&Search"));
QHBoxLayout* undo_lay = new QHBoxLayout();
this->undo_btn = new QPushButton(QObject::tr("U&ndo"));
this->shuffle_btn = new QPushButton(QObject::tr("S&huffle"));
this->refresh_btn = new QPushButton(QObject::tr("Re&fresh"));
this->activities = new QListWidget();
main_lay->addLayout(left_lay);
main_lay->addLayout(right_lay);
title_lay->addWidget(lbl_title);
title_lay->addStretch();
title_lay->addWidget(txt_title);
desc_lay->addWidget(lbl_desc);
desc_lay->addStretch();
desc_lay->addWidget(txt_desc);
type_lay->addWidget(lbl_type);
type_lay->addStretch();
type_lay->addWidget(txt_type);
dur_lay->addWidget(lbl_dur);
dur_lay->addStretch();
dur_lay->addWidget(txt_dur);
op_lay->addWidget(add_btn);
op_lay->addWidget(upd_btn);
op_lay->addWidget(rm_btn);
fil_lay->addWidget(fil_desc_btn);
fil_lay->addWidget(fil_type_btn);
fil_lay->addWidget(fil_search_btn);
undo_lay->addWidget(refresh_btn);
undo_lay->addWidget(shuffle_btn);
undo_lay->addWidget(undo_btn);
right_lay->addLayout(title_lay);
right_lay->addLayout(desc_lay);
right_lay->addLayout(type_lay);
right_lay->addLayout(dur_lay);
right_lay->addLayout(op_lay);
right_lay->addLayout(fil_lay);
right_lay->addLayout(undo_lay);
QHBoxLayout* sort_lay = new QHBoxLayout();
this->sort_title_btn = new QPushButton(QObject::tr("SortByTit&le"));
this->sort_desc_btn = new QPushButton(QObject::tr("SortByDes&c"));
this->sort_type_btn = new QPushButton(QObject::tr("SortByT&ype"));
left_lay->addWidget(activities);
left_lay->addLayout(sort_lay);
sort_lay->addWidget(sort_title_btn);
sort_lay->addWidget(sort_desc_btn);
sort_lay->addWidget(sort_type_btn);
this->refresh_list();
}
void GUI::connect_gui()
{
QObject::connect(add_btn, SIGNAL(clicked()), this, SLOT(add()));
QObject::connect(upd_btn, SIGNAL(clicked()), this, SLOT(edit()));
QObject::connect(rm_btn, SIGNAL(clicked()), this, SLOT(remove()));
QObject::connect(sort_title_btn, SIGNAL(clicked()), this, SLOT(search()));
QObject::connect(undo_btn, SIGNAL(clicked()), this, SLOT(undo()));
QObject::connect(refresh_btn, SIGNAL(clicked()), this, SLOT(refresh_list()));
QObject::connect(fil_search_btn, SIGNAL(clicked()), this, SLOT(search()));
// QObject::connect(sort_title_btn, SIGNAL(clicked()), this, SLOT(sort_title()));
QObject::connect(sort_type_btn, SIGNAL(clicked()), this, SLOT(sort_type()));
QObject::connect(sort_desc_btn, SIGNAL(clicked()), this, SLOT(sort_desc()));
QObject::connect(shuffle_btn, SIGNAL(clicked()), this, SLOT(shuffle()));
QObject::connect(fil_desc_btn, SIGNAL(clicked()), this, SLOT(filter_desc()));
QObject::connect(fil_type_btn, SIGNAL(clicked()), this, SLOT(filter_type()));
}
Main problem is the button "sort_title_btn" which when connected to an empty method or not at all connected gives me the error when clicked.
The creation of the project was done by:
qmake -project
Editing the .pro file to add QT += core gui widgets and to include the folder with headers.
qmake
And finally: make
Compiles nicely, all buttons work except "sort_title_btn" even when NOT connected to a SLOT and "search" if the field by which the search has to be made is empty, that's not a problem for it can be fixed easily.
So, what is there to be done for the "sort_title_btn"?
I tried to put it to another SLOT, it works. Changing the name of the function I connect it to seems to bug out or something giving me the error.