Combobox style error when using asynchronous loader

When asynchonously loading a component which contains a ComboBox{} element then the following error is produced: "file:///E:/qt/qt56dev/built/qml/QtQuick/Controls/ComboBox.qml:564: TypeError: Cannot read property '__dropDownStyle' of null". When "asynchronous: false" then the error is not produced. The error causes incorrect drawing of the ComboBox (with default and flat styles) and occurs randomly.

Button {
	onClicked: loader.source = "qrc:/qml/SamplePage.qml";
}

StackView {
	id: stackView
}


Loader {
	id: loader
	asynchronous: true

	onLoaded: {
		stackView.push(loader.item);
	}
}


Debug output:
file:///E:/qt/qt56dev/built/qml/QtQuick/Controls/ComboBox.qml:564: TypeError: Cannot read property '__dropDownStyle' of null