在写一个很简单的实验程序，我想实现的无非是，当我点击next button时，中央的label里面的内容发生变化，不知道为什么编译能通过，但是无法实现，当我点击next button时还报Exception at 0xxxxxxxx, code: 0xc0000005: read access violation at: 0x0, flags=0x0

下面贴上相关代码：

 /*experiment.h*/
#define initial_state 0
#define start_state 1
#define detect_state 2
#define pause_state 3
#define final_state 4
#pragma comment(lib, "Qt5Widgets.lib")

class Experiment:public QDialog{
    Q_OBJECT

public:
    Experiment(QWidget *parent=0);

protected:
    void closeEvent(QCloseEvent *event);

private slots:
    void ButtonFunction();

private:
    //Thread cv;
    //Thread mind;
    QLabel *direction;
    QPushButton *next;
    int state;
};

#endif // EXPERIMENT_H

 /*experiment.cpp*/
#include "experiment.h"
Experiment::Experiment(QWidget *parent): QDialog(parent){
    state=initial_state;
    QFont font("Courier",48,75,0);
    direction=new QLabel(tr("This is a experiment"),this);
    direction->setGeometry(450,400,760,73);
    direction->setFont(font);
    next=new QPushButton(tr("Next"),this);
    next->setGeometry(1500,850,71,21);
    next->setDefault(true);
    connect(next, SIGNAL(clicked()), this, SLOT(ButtonFunction()));
}

void Experiment::ButtonFunction(){
    switch(state){
    case initial_state:
        //TODO:
        next->setText(tr("please do something change"));//完全不起作用
        direction->setGeometry(450,400,1400,73);**
        break;
    case start_state:
        //TODO:
        break;
    case detect_state:
        //TODO:
        break;
    case pause_state:
        //TODO:
        break;
    case final_state:
        //TODO:
        break;
    }
}