Error while promoting Widget container in Qt - c++

Question

I've created my own class in QT 5.4 with header and source file, then I've implemented there a video player. The problem is, that it's working fine when I'm placing it in the center of the main window or when I'm showing it in a separate window, but when I'm trying to create a Widget container in the main window and promote widget to it, it gives me some errors:
'videowindow' does not name a type
'widget' was not declared in this scope
expected type-specifier before 'videowindow'
So, here is the code:
videowindow.h - header of my implemented class
#ifndef VIDEOWINDOW_H
#define VIDEOWINDOW_H
#include "mainwindow.h"
class videoWindow
{
private:
QMediaPlayer *player;
QMediaPlaylist *playlist;
public:
videoWindow();
void video_enable();
void widget();
};
#endif // VIDEOWINDOW_H
videowindow.cpp - source
#include "videowindow.h"
#include "mainwindow.h"
videoWindow::videoWindow()
{
}
void videoWindow::video_enable()
{
player = new QMediaPlayer;
playlist = new QMediaPlaylist;
playlist->addMedia(QUrl::fromLocalFile("D:\\фото\\video\\P1150277.MOV"));
QVideoWidget *videoWid = new QVideoWidget;
player->setVideoOutput(videoWid);
videoWid->show();
player->setPlaylist(playlist);
playlist->setCurrentIndex(1); // start from a first video in the playlist
player->play();
}
mainwindow.h:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QtMultimedia>
#include <QtMultimediaWidgets>
#include <QtDebug>
#include <string>
#include <QGraphicsVideoItem>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
private:
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "videowindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}
MainWindow::~MainWindow()
{
delete ui;
}
main.cpp
#include "mainwindow.h"
#include "videowindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
InstantPlayer.pro - .pro file of the project
#-------------------------------------------------
#
# Project created by QtCreator 2016-06-24T11:37:26
#
#-------------------------------------------------
QT += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = InstantPlayers
TEMPLATE = app
#
QT += multimedia
QT += multimediawidgets
#
SOURCES += main.cpp\
mainwindow.cpp \
videowindow.cpp \
playerbase.cpp
HEADERS += mainwindow.h \
videowindow.h \
playerbase.h
FORMS += mainwindow.ui \
playerbase.ui