Qt: QWidget::paintEngine: Should no longer be called

I'm trying to make an app where you can draw with your finger on a canvas.
To achieve this, I'm subclassing QWidget as MFCanvas, registered the class in QML with
qmlRegisterType<>(), implementing the virtual paintEvent(); function, and
drawing on it with a QPainter inside the paintEvent(); function.

The Problem:
Upon construction, the QPainter throws this warning:

QWidget::paintEngine: Should no longer be called

Then, serveral other related warnings are thrown:

QPainter::begin: Paint device returned engine == 0, type: 1QPainter::setPen: Painter not active

No wonder: the QPainter didn't draw anything...
Also, am i supposed to call paintEvent(); by myself?
Or should it be called every frame by QWidget, and i somehow messed it up?

I searched the web, but all posts i found had either no answer to them, or they where
using something else than QWidget.

My Code:

mfcanvas.cpp:

#include "mfcanvas.h"#include <QDebug>#include <QPainter>#include <QVector2D>#include <QList>MFCanvas::MFCanvas(QWidget *parent) : QWidget(parent){    paths = new QList<QList<QVector2D>*>();    current = NULL;    QWidget::resize(100, 100);}MFCanvas::~MFCanvas(){    delete paths;}void MFCanvas::paintEvent(QPaintEvent *){    if(current!=NULL){        if(current->length() > 1){            QPainter painter(this);            painter.setPen(Qt::black);            for(int i = 1; i < current->length(); i++){                painter.drawLine(current->at(i-1).x(), current->at(i-1).y(), current->at(i).x(), current->at(i).y());            }        }    }}void MFCanvas::pressed(float x, float y){    if(current==NULL){        qDebug() << "null:"<<current;        current = new QList<QVector2D>();        current->append(QVector2D(x, y));    }else{        qDebug() << "current:"<<current;    }    paintEvent(NULL);}void MFCanvas::update(float x, float y){    current->append(QVector2D(x, y));}void MFCanvas::resize(int w, int h){    QWidget::resize(w, h);}

main.cpp:

#include <QApplication>#include <QQmlApplicationEngine>#include <QtQml>#include <QSurfaceFormat>#include "creator.h"#include "mfcanvas.h"int main(int argc, char *argv[]){    QApplication app(argc, argv);    qmlRegisterType<MFCanvas>("com.cpp.mfcanvas", 1, 0, "MFCanvas");    QQmlApplicationEngine engine;    QQmlComponent *component = new QQmlComponent(&engine);    QObject::connect(&engine, SIGNAL(quit()), QCoreApplication::instance(), SLOT(quit()));    Creator creator(component);    QObject::connect(component, SIGNAL(statusChanged(QQmlComponent::Status)), &creator, SLOT(create(QQmlComponent::Status)));    component->loadUrl(QUrl("qrc:///main.qml"));    int rv;    rv = app.exec();    delete component;    return rv;}

creator.cpp:

#include "creator.h"#include <QQuickWindow>#include <QDebug>Creator::Creator(QQmlComponent *component){    this->component = component;}void Creator::create(QQmlComponent::Status status){    if(status == QQmlComponent::Ready){        QObject *topLevel = component->create();        QQuickWindow::setDefaultAlphaBuffer(true);        QQuickWindow *window = qobject_cast<QQuickWindow *>(topLevel);        QSurfaceFormat surfaceFormat = window->requestedFormat();        window->setFormat(surfaceFormat);        window->show();    }}

main.qml: (the important part)

import QtQuick 2.2import QtQuick.Controls 1.1import QtQuick.Controls.Styles 1.2import QtQuick.Layouts 1.1import QtQuick.Window 2.0import com.cpp.mfcanvas 1.0ApplicationWindow {    visible: true    width: 640    height: 480    title: qsTr("MFCanvas")    onSceneGraphInitialized: {        drawMenu.visible = true;        lineWidth.visible = true;        colorMenu.visible = true;        drawMenu.visible = false;        lineWidth.visible = false;        colorMenu.visible = false;    }    Rectangle {        id: main        anchors.fill: parent        property real toolsH: 15        property real iconW: 25        property real menuH: 8        property real menuW: 16        property real dpi: (Screen.logicalPixelDensity == undefined ? 6 : Screen.logicalPixelDensity) * 1.5        property color choosenColor: Qt.hsla(hue.value, saturation.value, luminance.value, 1)        Text {            anchors.centerIn: parent            font.pointSize: 60            text: "MFCanvas"        }        MFCanvas {            id: canvas            Component.onCompleted: {                canvas.resize(main.width, main.height);            }        }    //...    }}

Tell me if you need any additional information.
Thank you in advance! =)