QML QT的grabToImage获取图像字节

For some reason, saving a png back out from qml directly doesn't work. I have a qml UI on top of a Golang application. When I do

source.grabToImage(function(result){
    console.log("image: ", result.url)
if (!result.saveToFile(urlNoProtocol)){
    console.error('Unknown error saving to',urlNoProtocol);
} else {
    console.log("saved to " + urlNoProtocol)
}

I get an error saving out. The location to save the file is coming from a fileDialog and I preprocess it to remove the file:// which I understand needs to be removed before using saveToFile. However I get an unknown error saving from the above code.

I suspect this is something to do with the qml being embedded in the binary application (perhaps to do with qrc:// or something)

Anyway my current plan is to send the image to the golang backend and save it out from there, so my question is, how from grabToImage or saveToFile can I get the image bytes that I will then save?

N.B Using therecipe/qt for the interface between Golang and qml

I have used this method for grabbing screen shots..

function performScreenShot(item, name) {
    if(typeof(item) === "undefined") {
        return;
    }
    else if(typeof(name) !== "string") {
        name = "screenshot.png"
    }

    item.grabToImage(function(result) {
        result.saveToFile(name);
    });
}

Usage example:-

performScreenShot(main, "screenshot-" + screenShotIndex + ".png")

Where main is my object id and screenShotIndex is something im incrementing so as not to override any previous capture.

Ensure that you have write permission on the device.