Qt 创建者 C++。引用未解析的外部符号

//mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QGraphicsView>
#include "graphicsscene.h"

namespace Ui
{
class MainWindow;
}


class MainWindow : public QMainWindow
{
    Q_OBJECT
private:
    QGraphicsView *graph;
    QWidget *buttonsWidget;
    QWidget *statusWidget;
    CustomScene *scene;


public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;



};


#endif // MAINWINDOW_H

//mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    graph = ui->graphicsView; // Initialize pointer for graphics view
    scene = new CustomScene(); // Тут ошибка
    scene->setSceneRect(ui->graphicsView->rect()); // Set size and positon for scene
    scene->addLine(0,0,200,200); // Test
    graph->setScene(scene); // Set scene on graphics view

    statusWidget = ui->StatusWidget; // Initialize pointer for own widget "Status Widget"

}

MainWindow::~MainWindow()
{
    delete scene;
    delete ui;
}

//graphicsscene.h

#ifndef GRAPHICSSCENE_H
#define GRAPHICSSCENE_H

#include <QObject>
#include <QGraphicsScene>
#include <QGraphicsSceneMouseEvent>


class CustomScene : public QGraphicsScene
{
    Q_OBJECT
public:
    explicit CustomScene(QObject *parent = 0);
    ~CustomScene();

signals:
    // Сигнал для передачи координат положения курсора мыши
    void signalTargetCoordinate(QPointF point);

public slots:

private:
    // Функция, в которой производится отслеживание положения мыши
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
};

#endif // GRAPHICSSCENE_H

//graphicsscene.cpp
#include "graphicsscene.h"

CustomScene::CustomScene(QObject *parent) :
    QGraphicsScene()
{
    Q_UNUSED(parent);
}

CustomScene::~CustomScene()
{

}

void CustomScene::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
    emit signalTargetCoordinate(event->scenePos());
}

给出错误消息：

mainwindow.obj:-1：错误：LNK2019：未解析的外部符号引用“public: __thiscall CustomScene::CustomScene(class QObject *)”(??0CustomScene@@QAE@PAVQObject@@@Z) 在函数“public: __thiscall MainWindow”中::MainWindow(class QWidget *)" (??0MainWindow@@QAE@PAVQWidget@@@Z)`

如果你双击它，它会写Файл не найден в:mainwindow.obj.

如果我会在mainwindow.h我的声明CustomScene并在 中定义mainwindow.cpp，那么没有错误，但我如何确保其他文件中有 CustomScene？