Implementing autofill for onlyfans login page with QWebEngine based app - c++

Question

I'm trying to implement autofill functionality in a Qt Webengine based app.
My approach is based off Viper browser's implementation: running script on page's loadFinished signal. The script run just looks up all the fields that can be populated and fills them as simple as elem.value = 'value'.
It works with simple example like e.g. my wifi router config page (which uses very simple logic for forms). But it doesn't work with e.g. https://onlyfans.com/ login page.
When I run my script I get JS errors like:
"js: Uncaught TypeError: Cannot set property 'value' of null".
The minimal example demonstrating the issue:
#include <QApplication>
#include <QObject>
#include <QTimer>
#include <QWebEngineView>
#include <QWebEnginePage>
#include <QWebEngineProfile>
#include <QWebEngineScript>
#include <QWebEngineScriptCollection>
const char* URL_ONLYFANS = "https://onlyfans.com/";
const char* URL_ROUTER = "http://192.168.0.1/";
const char* SCRIPT_ROUTER(R"(
document.querySelector('input[type=text]').value = 'admin';
document.querySelector('input[type=password]').value = 'mypass';
)");
const char* SCRIPT_ONLYFANS(R"(
document.querySelector('input[name=email]').value = "random#gmail.com";
document.querySelector('input[name=password]').value = "randompass";
)");
class Observer : public QObject
{
Q_OBJECT
public:
explicit Observer(QWebEnginePage *page): _page(page)
{}
public slots:
void onPageLoad(bool ok)
{
qDebug() << "Page loaded" << ok;
// Running script right away pretty much guarantees JS errors
// No JS errors if delay is >= 1500 ms:
QTimer::singleShot(1500, this, &Observer::runScript);
qDebug() << "Scheduled running script";
}
void runScript()
{
_page->runJavaScript(SCRIPT_ONLYFANS);
qDebug() << "Ran script";
}
private:
QWebEnginePage *_page{nullptr};
};
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QWebEngineView view;
QWebEngineProfile profile;
QWebEnginePage page(&profile);
view.setPage(&page);
view.setUrl(QUrl(URL_ONLYFANS));
view.resize(1024, 750);
Observer observer(view.page());
QObject::connect(view.page(), &QWebEnginePage::loadFinished,
&observer, &Observer::onPageLoad);
/*
{
QWebEngineScript script;
script.setName("autofill");
script.setSourceCode(SCRIPT_ONLYFANS);
// Also tried `DocumentCreation` and `Deferred`, didn't work for Onlyfans
script.setInjectionPoint(QWebEngineScript::DocumentReady);
script.setRunsOnSubFrames(true);
script.setWorldId(QWebEngineScript::ApplicationWorld);
view.page()->scripts().insert(script);
}
*/
view.show();
return app.exec();
}
#include "main.moc"
As can be seen I tried to insert my scripts in two ways: on page's loadFinished signal (both immediately and with a time delay) and by inserting my script into page's scripts collection. I tried all available insertion points: DocumentCreation, DocumentReady and Deferred.
In all cases I get JS error shown above. Except for the case when I delay script execution by about 1500 ms, which is obviously a guess and is not robust.
I do understand that the elements I'm trying to set are probably not yet created, so I'd like some advice on how to properly time the script execution.