//texteditor.h
#ifndef TEXTEDITOR_H
#define TEXTEDITOR_H

#include <QMainWindow>
#include <QtCore>
#include <QtGui>
#include <QFileDialog>

QT_BEGIN_NAMESPACE
namespace Ui { class TextEditor; }
QT_END_NAMESPACE
class TextEditor : public QMainWindow
{
    Q_OBJECT

public:
    TextEditor(QWidget *parent = nullptr);
    ~TextEditor();
private slots:
    void on_Save_triggered();
private:
    Ui::TextEditor *ui;
    QString mFilename;
};
#endif // TEXTEDITOR_H



//texteditor.cpp
#include "texteditor.h"
#include "ui_texteditor.h"
#include "about.h"
#include "save.h"


TextEditor::TextEditor(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::TextEditor)
{
    ui->setupUi(this);
    this->setCentralWidget(ui->textEdit);
}

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

void TextEditor::on_Save_triggered()
{
    QFile sFile(mFilename);
    if(sFile.open(QFile::WriteOnly | QFile::Text))
    {
        QTextStream out(&sFile);
        out << ui->textEdit->toPlainText();
        sFile.flush();
        sFile.close();
    }
}
//save.h
#ifndef SAVE_H
#define SAVE_H

#include <QMainWindow>
#include <QtCore>
#include <QtGui>
#include <QFileDialog>

namespace Ui {
class Save;
}
class Save : public QDialog
{
    Q_OBJECT
public:
    explicit Save(QWidget *parent = nullptr);
    ~Save();
private slots:
    void on_pushButton_clicked();
private:
    Ui::Save *ui;
};
#endif // SAVE_H



//save.cpp
#include "save.h"
#include "ui_save.h"
#include "texteditor.h"

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

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

void Save::on_pushButton_clicked()
{
    on_Save_triggered(); //сдесь этая ошибка
}