Problems in creating a Qt Widget Based Application: Invalide use of incomplete type

I followed the instruction on the Qt website step by step to create my first widget based application.

In the main file I have

#include "textfinder.h"
#include <QApplication>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    TextFinder w;
    w.show();
    return a.exec();
}

In the Header file:

#ifndef TEXTFINDER_H
#define TEXTFINDER_H
#include <QWidget>

QT_BEGIN_NAMESPACE
namespace Ui { class TextFinder; }
QT_END_NAMESPACE

class TextFinder : public QWidget
{
    Q_OBJECT
public:
    TextFinder(QWidget *parent = nullptr);
    ~TextFinder();

private slots:
    void on_pushButton_clicked();

private:
    Ui::TextFinder *ui;
    void loadTextFile();
};
#endif // TEXTFINDER_H

and in the source file I have

#include "textfinder.h"
#include "./ui_textfinder.h"
#include <QFile>
#include <QTextStream>

TextFinder::TextFinder(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::TextFinder)
{
    ui->setupUi(this);
    loadTextFile();
}

TextFinder::~TextFinder()
{
    delete ui;
}


void TextFinder::on_pushButton_clicked()
{
    QString searchString = ui->lineEdit->text();
    ui->textEdit->find(searchString, QTextDocument::FindWholeWords);
}

void TextFinder::loadTextFile()
{
    QFile inputFile(":/input.txt");
    inputFile.open(QIODevice::ReadOnly);

    QTextStream in(&inputFile);
    QString line = in.readAll();
    inputFile.close();

    ui->textEdit->setPlainText(line);
    QTextCursor cursor = ui->textEdit->textCursor();
    cursor.movePosition(QTextCursor::Start, QTextCursor::MoveAnchor, 1);
}

In the ``textfinder.ui``` file I have the following

The projects sources in the CMakeLists.txt file are

set(PROJECT_SOURCES
  main.cpp
  textfinder.cpp
  textfinder.h
  textfinder.ui
  ${TS_FILES}
  textfinder.qrc
)

If I press run now I get the error message invalid use of incomplete type 'class Ui::TextFinder'. But I thought I do not have to care for the ui_textfinder.h file since this will be autogenerated. By reading the other questions/answers I think it could have to do with the naming. But I can not understand where my mistake is since I followed the tutorial carefully.

EDIT-------- The missing ui_textfinder.h file -------

#ifndef UI_TEXTFINDER_H
#define UI_TEXTFINDER_H
#include <QtCore/QVariant>
#include <QtWidgets/QApplication>
#include <QtWidgets/QHBoxLayout>
#include <QtWidgets/QLabel>
#include <QtWidgets/QLineEdit>
#include <QtWidgets/QPushButton>
#include <QtWidgets/QTextEdit>
#include <QtWidgets/QVBoxLayout>
#include <QtWidgets/QWidget>

QT_BEGIN_NAMESPACE

class Ui_findButton
{
    public:
    QWidget *widget;
    QVBoxLayout *verticalLayout;
    QHBoxLayout *horizontalLayout;
    QLabel *label;
    QLineEdit *lineEdit;
    QPushButton *pushButton;
    QTextEdit *textEdit;

    void setupUi(QWidget *findButton)
    {
        if (findButton->objectName().isEmpty())
         findButton->setObjectName(QString::fromUtf8("findButton"));
        findButton->resize(800, 600);
        widget = new QWidget(findButton);
        widget->setObjectName(QString::fromUtf8("widget"));
        widget->setGeometry(QRect(170, 140, 298, 227));
        verticalLayout = new QVBoxLayout(widget);
        verticalLayout->setObjectName(QString::fromUtf8("verticalLayout"));
        verticalLayout->setContentsMargins(0, 0, 0, 0);
        horizontalLayout = new QHBoxLayout();
        horizontalLayout->setObjectName(QString::fromUtf8("horizontalLayout"));
        label = new QLabel(widget);
        label->setObjectName(QString::fromUtf8("label"));

        horizontalLayout->addWidget(label);

        lineEdit = new QLineEdit(widget);
        lineEdit->setObjectName(QString::fromUtf8("lineEdit"));

        horizontalLayout->addWidget(lineEdit);

        pushButton = new QPushButton(widget);
        pushButton->setObjectName(QString::fromUtf8("pushButton"));

        horizontalLayout->addWidget(pushButton);


        verticalLayout->addLayout(horizontalLayout);

        textEdit = new QTextEdit(widget);
        textEdit->setObjectName(QString::fromUtf8("textEdit"));

        verticalLayout->addWidget(textEdit);
        retranslateUi(findButton);

        QMetaObject::connectSlotsByName(findButton);
    } // setupUi

    void retranslateUi(QWidget *findButton)
    {
        findButton->setWindowTitle(QCoreApplication::translate("findButton", "TextFinder", nullptr));
    label->setText(QCoreApplication::translate("findButton", "Keyword", nullptr));
    pushButton->setText(QCoreApplication::translate("findButton", "Find", nullptr));
} // retranslateUi

};

namespace Ui {
    class findButton: public Ui_findButton {};
} // namespace Ui

QT_END_NAMESPACE

#endif // UI_TEXTFINDER_H