Linking error: LNK2019 & LNK1120 - when creating/connecting a SIGNAL

My goal here is to create a signal "expandParentItem" in my QFileSystemModel subclass so that when a parent item is checked in the associated QtreeView, the QtreeView can automatically expand it to show its children.
I define and I emit the signal like this:

FileSysSelectModel.h

Code:

#ifndef FILESYSSELECTMODEL_H
#define FILESYSSELECTMODEL_H
 
#include<QFileSystemModel>
#include <QSet>
#include <QPersistentModelIndex>
#include <QVariant>
 
class FileSysSelectModel: public QFileSystemModel
{
private:
    QSet<QPersistentModelIndex> m_checkTable;
    QVariant m_noData;
 
    void FileSysSelectModel::updateSubMembers(const QModelIndex &index, const QVariant &value);
 
public:
    int FileSysSelectModel::columnCount(const QModelIndex &parent) const;
    QVariant FileSysSelectModel::data(const QModelIndex &index, int role) const;
    virtual Qt::ItemFlags FileSysSelectModel::flags(const QModelIndex &index) const;
    virtual bool FileSysSelectModel::setData(const QModelIndex &index, const QVariant &value, int role);
 
signals:
    void expandParentItem(const QModelIndex &index);
};
 
#endif // FILESYSSELECTMODEL_H

Function managing the signal in FileSysSelectModel.cpp

Code:

//if the current item has children:
// check all the child items if the parent item is checked.
// uncheck all the child items if the parent item is unchecked.
void FileSysSelectModel::updateSubMembers(const QModelIndex &index, const QVariant &value)
    {
         QModelIndex firstChild = index.child(0,0);
         QModelIndex currentChild;
        if (firstChild.isValid())
        {
            if(value == Qt::Checked) emit expandParentItem(index);
 
            int i = 0;
            while(1)
            {
                currentChild = index.child(i,0);
               //Recursive call
                FileSysSelectModel::updateSubMembers(currentChild, value);
                if (!currentChild.isValid()) break;
                if(value == Qt::Checked) m_checkTable.insert(currentChild); else m_checkTable.remove(currentChild);
                i = i+1;
            }
            emit dataChanged(firstChild, currentChild);
        }
    }

I connect the signal to the QtreeView in my main window like this:

mainwindow.cpp

Code:

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
 
    m_fileSystemModel = new FileSysSelectModel;
    m_fileSystemModel->setRootPath(QDir::currentPath());
    ui->scriptFileView->setModel(m_fileSystemModel);
 
    QObject::connect(m_fileSystemModel,SIGNAL(expandParentItem(QModelIndex)),ui->scriptFileView,SLOT(expand(QModelIndex)));
}
 
MainWindow::~MainWindow()
{
    delete ui;
}

When I try to compile, I get the following error message:

FileSysSelectModel.obj:-1: error: LNK2019: unresolved external symbol "public: void __thiscall FileSysSelectModel::expandParentItem(class QModelIndex const &)" (?expandParentItem@FileSysSelectModel@@QAEXABVQMod elIndex@@@Z) referenced in function "private: void __thiscall FileSysSelectModel::updateSubMembers(class QModelIndex const &,class QVariant const &)" (?updateSubMembers@FileSysSelectModel@@AAEXABVQMod elIndex@@ABVQVariant@@@Z)

debug\Designer_test.exe:-1: error: LNK1120: 1 unresolved externals

I'm new to Qt and I'm having a hard time understanding this error message.
Could someone help me with this issue?