Unable to create WebPlugin for pdf extension file

ASSERTION FAILED: !widget || widget->isPluginView()
C:\QtCustom\Qt5.4.0\5.4\Src\qtwebkit\Source\WebCore/plugins/PluginView.h(458) : WebCore::toPluginView

0	WTFCrash	Assertions.cpp	345	0x11139ab7	
1	WebCore::toPluginView	PluginView.h	459	0xfd2804e	
2	WebCore::FrameLoaderClientQt::redirectDataToPlugin	FrameLoaderClientQt.cpp	1562	0xfd24c50	
3	WebCore::PluginDocumentParser::appendBytes	PluginDocument.cpp	132	0x105f19fb	
4	WebCore::DocumentWriter::addData	DocumentWriter.cpp	226	0x104e887f	
5	WebCore::DocumentLoader::commitData	DocumentLoader.cpp	804	0x101bc0f8	
6	WebCore::FrameLoaderClientQt::committedLoad	FrameLoaderClientQt.cpp	882	0xfd22835	
7	WebCore::DocumentLoader::commitLoad	DocumentLoader.cpp	746	0x101bc747	
8	WebCore::DocumentLoader::dataReceived	DocumentLoader.cpp	856	0x101bd983	
9	WebCore::CachedRawResource::notifyClientsDataWasReceived	CachedRawResource.cpp	110	0x109b7716	
10	WebCore::CachedRawResource::addDataBuffer	CachedRawResource.cpp	67	0x109b6ec0	
11	WebCore::SubresourceLoader::didReceiveDataOrBuffer	SubresourceLoader.cpp	251	0x10901688	
12	WebCore::SubresourceLoader::didReceiveData	SubresourceLoader.cpp	227	0x10900e9d	
13	WebCore::ResourceLoader::didReceiveData	ResourceLoader.cpp	476	0x1062927b	
14	WebCore::QNetworkReplyHandler::forwardData	QNetworkReplyHandler.cpp	691	0x1059d4f8	
15	WebCore::QNetworkReplyHandlerCallQueue::flush	QNetworkReplyHandler.cpp	256	0x1059c394	
16	WebCore::QNetworkReplyHandlerCallQueue::unlock	QNetworkReplyHandler.cpp	237	0x1059c339	
17	WebCore::QueueLocker::~QueueLocker	QNetworkReplyHandler.cpp	264	0x1059fae1	
18	WebCore::QNetworkReplyWrapper::emitMetaDataChanged	QNetworkReplyHandler.cpp	402	0x1059ce06	
19	WebCore::QNetworkReplyWrapper::receiveSniffedMIMEType	QNetworkReplyHandler.cpp	366	0x1059cbb9	
...	<plus>				

#include <QWebPluginFactory>

class MyWebPluginFactory : public QWebPluginFactory
{
public:
    MyWebPluginFactory(QObject *parent);

protected:
    QObject* create(
            const QString &mimeType, const QUrl &url, const QStringList &argumentNames, const QStringList &argumentValues) const;
    QList<Plugin> plugins() const;
};

#include "mywebpluginfactory.h"

MyWebPluginFactory::MyWebPluginFactory(QObject *parent) :
    QWebPluginFactory(parent)
{}

QList<QWebPluginFactory::Plugin> MyWebPluginFactory::plugins() const
{
    QWebPluginFactory::MimeType mimeType;
    mimeType.name = "application/pdf";
    mimeType.description = "PDF document";
    mimeType.fileExtensions = QStringList() << "pdf";

    QWebPluginFactory::Plugin plugin;
    plugin.name = "PDF file viewer";
    plugin.description = "A PDF file plugin.";
    plugin.mimeTypes = QList<MimeType>() << mimeType;

    return QList<QWebPluginFactory::Plugin>() << plugin;
}

QObject *MyWebPluginFactory::create(
            const QString &mimeType, const QUrl &, const QStringList &, const QStringList &) const
{
    if (mimeType == "application/pdf")
    {
        QWidget *myWidget = new QWidget();
        return myWidget;
    }

    return nullptr;
}

MyWebPluginFactory *pluginFactory = new MyWebPluginFactory(this);
page()->setPluginFactory(pluginFactory);