"QPixmap: Must construct a QGuiApplication before a QPixmap" Debug Error!

While running our app developed on Windows 10 and closing the gui window with the "X" button we get a Microsoft Visual C++ Runtime Library Error (see attachment).
When running in debug mode an exception is triggered in the QApplication::exec() method.

Attachment 13218

This is our main function:

Code:

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    GeDetektEventFilter filter;
    app.installNativeEventFilter(&filter);
 
    std::unique_ptr<Sentinel>& sentinel = Sentinel::getInstance();
    sentinel->connectThreads();
    sentinel->createAppFiles();
    sentinel->startQmlEngine();
 
    sentinel->startHardware();
 
    manager::connections::connectAll();
 
    //Uncomment this to deactivate tests (not recommended)
    ::testing::InitGoogleTest(&argc, argv);
    RUN_ALL_TESTS();
 
    return qApp->exec();
}

Our Gui is created with QtQuick and qml.

Code:

import QtQuick 2.2
import QtQuick.Window 2.12
import QtQuick.Controls 1.4
import QtQuick.Layouts 1.12
 
import DisplayInformation 1.0
 
Window
{
    id: mainWindow
 
    visible: true
    width: displayInformation.WINDOW_SIZE.width + displayInformation.SIDE_BAR_WIDTH
    height: displayInformation.WINDOW_SIZE.height
    title: qsTr("Sentinel")
 
    maximumHeight: height
    maximumWidth: width
    minimumHeight: height
    minimumWidth: width
 
    //Old Qt Bug Displays error here although functional
    onClosing:
    {
        sentinel.stopComClient();
    }
 
    Rectangle
    {
        id: mainArea
        Keys.onPressed:
        {
            if (event.key === Qt.Key_F9)
            {
                sidebarLoader.source = ""
                qmlEngine.clearCache()
                sidebarLoader.source = "Sidebar.qml"
 
                imageDisplayLoader.source = ""
                qmlEngine.clearCache()
                imageDisplayLoader.source = "ImageDisplay.qml"
 
                if (mainArea.logOpen === true)
                {
                    mainArea.logWindow.close()
                    mainArea.logComponent.source = ""
                    qmlEngine.clearCache()
                    mainArea.logComponent.source = "LogWindow.qml"
                }
 
                console.log("Sources Reloaded");
 
                event.accepted = true;
            }
 
            if (event.key === Qt.Key_L && logOpen === false)
            {
                sentinel.startHardware()
 
                console.log("Log Window")
                logComponent = Qt.createComponent("LogWindow.qml")
                logWindow = logComponent.createObject(mainWindow)
 
                logWindow.show()
                logOpen = true
            }
        }
 
        Connections
        {
            target: mainArea.logWindow
            onClosing:
            {
                console.log("Log Window closed")
                mainArea.logOpen = false
            }
        }
 
        property bool scanAvailable: false
        property bool scanStopped: true
        property bool logOpen: false
        property Window logWindow
        property Component logComponent
    }
 
    RowLayout
    {
        id: rowLayout
        anchors.fill: parent
 
        Loader
        {
            id: imageDisplayLoader
            source: "ImageDisplay.qml"
        }
 
        Loader
        {
            id: sidebarLoader
            Layout.fillHeight: true
            Layout.fillWidth: true
            source: "Sidebar.qml"
        }
    }
}

The qml files are loaded in following function over an environment variable:

Code:

void Sentinel::startQmlEngine()
{
    qmlRegisterType<gui::DisplayInformation>("DisplayInformation", 1, 0, "DisplayInformation");
    qmlRegisterType<gui::ImageItem>("ImageItem",1,0,"ImageItem");
    qmlRegisterType<gui::ImageProvider>("ImageProvider", 1, 0, "ImageProvider");
 
    m_qmlEngine.rootContext()->setContextProperty("displayInformation",
                                                  &m_imageProvider.getDisplayInformation());
 
    m_qmlEngine.rootContext()->setContextProperty("sentinel", this);
    m_qmlEngine.rootContext()->setContextProperty("imageProvider", &m_imageProvider);
    m_qmlEngine.rootContext()->setContextProperty("scanArea", modules::ScanArea::getInstance().get());
    m_qmlEngine.rootContext()->setContextProperty("comClient", modules::ComClient::getInstance().get());
    m_qmlEngine.rootContext()->setContextProperty("ptu", modules::PtuControl::getInstance().get());
    m_qmlEngine.rootContext()->setContextProperty("logOutput", helpers::LogOutput::getInstance().get());
    m_qmlEngine.rootContext()->setContextProperty("qmlEngine", &m_qmlEngine);
 
    m_qmlEngine.load(qEnvironmentVariable("MAIN_QML"));
    assert(!m_qmlEngine.rootObjects().isEmpty());
 
    //These are the connections interfacing module state to gui (image processing methods and qml)
    using namespace modules;
 
    QObject::connect(&m_imageProvider,
                     &gui::ImageProvider::backgroundChanged,
                     this, &Sentinel::updateBackgroundS);
 
    QObject::connect(ComClient::getInstance().get(),
                     &ComClient::compoundsNameAndStateChanged,
                     this, &Sentinel::updateCompoundsS);
 
    QObject::connect(GigECamera::getInstance().get(),
                     &GigECamera::newFrameAvailable,
                     &m_imageProvider,
                     &gui::ImageProvider::updateBackgroundImage);
 
    QObject::connect(ToxelBufferExtractor::getInstance().get(),
                     &ToxelBufferExtractor::updateToxelPixel,
                     &m_imageProvider,
                     &gui::ImageProvider::paintNewOverlayCircle);
}

I've researched for quite a bit to find a solution for this and have seen there were some reported bugs refering to this error. However, I am not sure if and when these bugs have been fixed or even apply to our problem.
There might also be something missing in the way we create and quit the app as this is a very general application scenario for a Qt app and quite a disturbing behaviour.

Very grateful for any suggestions.