I need help with an undeclared identifier error in C++ using QT Creator

Hello, I am creating my first program for Linux using C++ and QT Creator 4.11.0
I am new to QT Creator, and I am not extremely familiar with C++.I have been using Delphi to create programs for Windows since 1998.
At this point I have the basic framework for my program and it runs. I have a nice working UI, have learned signals and slots, created several slots, and have a button causing text to be written to a lineEdit.
I have one problem relating to an undeclared identifier error, that prevents the program from running, and also prevents me from making further progress on this program. There is a second related problem that apparently is due to a problem in the code QT Creator created for me. Also there is a problem with mainwindow.h that QT Creator also created for me.
I have included the code. This code includes MainWindow.h, Main.cpp and Mainwindow.cpp. The only thing I have removed is several empty slot functions that are identical to the ones you can see.

Regarding the first problem, the one that is preventing the program from running. Look at MainWindow.cpp. Everything here was written for me automatically by QT Creator except for the function RefreshDisplay(), my function that will display several variables in LineEdits, and the line in RefreshDisplay() and in MainWindow::on_pushButton_clicked(),
 that writes some text to a lineEdit. The other functions are slots created automatically for me. They will have some math added, and they call RefreshDisplay(); The first problem, is that the line that puts some text into the lineedit works from within the slot function created by QT Creator, but it doesn’t work from the function I wrote, RefreshDisplay(). Here it gets an undeclared identifier error for “ui”. I cannot figure out why this happens here in my function, and not in the slot functions. I think I need to declare ui for my function. But I don’t know where or how. I’ve tried several things that didn’t work. If someone can show me how to fix this, I will be able to move forward.

Regarding the second problem, for each of the slot functions created for me by QT Creator, there is an error message saying “Use of undeclared identifier ‘MainWindow’ ”. This is not preventing the program from running, but I would like to get rid of all these error messages if I can. I have no idea how to fix this, other than that MainWindow is not being properly declared, possibly in the header.

Finally, the third problem. There seems to be something wrong in the file MainWindow.h, which was automatically created for me by QT Creator. The cause of this problem may also be the cause of the second problem.
The first symptom of this problem is a message at the top of the MainWindow.cpp editing window in yellow that says: “Warning: This code model could not parse an included file, which might lead to incorrect code completion and highlighting, for example.”
The second symptom is the identical message, also in a yellow window at top of the MainWindow.h editor.
Finally, the third symptom: In MainWindow.h, which was written automatically by QT Creator, where it defines class MainWindow. That line has an error message saying: “Expected class name”. The next line just below { and above the public declaration, says QOBJECT. This line has four error messages. They are:
“Incomplete result type ‘QString’ in function definition”, “Calling ‘TR’ with incomplete return type ‘QString’.”, “Incomplete result type ‘QString” in function definition” and “Calling ‘TR’ with_”.

I don’t know how to fix this either. Perhaps the word QOBJECT needs a semicolon at the end.

I appreciate any help you can give me on this. Here is the code:

MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE
class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
private slots:
    void on_pushButton_2_clicked();
    void on_pushButton_clicked();
    void on_pushButton_3_clicked();
    void on_pushButton_4_clicked();
    void on_pushButton_13_clicked();
    void on_pushButton_14_clicked();
    void on_pushButton_11_clicked();
    void on_pushButton_12_clicked();
    void on_pushButton_5_clicked();
    void on_pushButton_6_clicked();
    void on_pushButton_7_clicked();
    void on_pushButton_8_clicked();
private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

Main.cpp

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

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}
MainWindow::~MainWindow()
{
    delete ui;
}
void RefreshDisplay()                           //Updates Displayed Variables
{
    //ui->lineEdit->setText("Boogeyman");         //this line causes compile                      //failure because of undeclared identifier "ui"
}
void MainWindow::on_pushButton_2_clicked()      //Total Frames Increase Button Slot
{
    RefreshDisplay();
}
void MainWindow::on_pushButton_clicked()        //Total Frames Decrease Button Slot
{
    ui->lineEdit->setText("Boogeyman");         //Prints to lineEdit successfully
}