I've only tested with Qt 5.4.1.

If you remove a sourceComponent from a loader, the component's parent is set to null before that component is destroyed. This leads to an awkward need for extra null checking code to avoid a console error message. In my real-world code where I first encountered this issue, I've not found that this null actually breaks anything important but the error message is concerning.

Consider the trivial QML below. When the user clicks the screen and the sourceComponent is set to null, I get the following in the console, where lines 13 and 14 refer to the "parent.width" and parent.height" expressions respectively:

file:///Users/xxx/LoaderTest/LoaderTest.qml:13: TypeError: Cannot read property of null
file:///Users/xxx/LoaderTest/LoaderTest.qml:14: TypeError: Cannot read property of null

import QtQuick 2.4

Rectangle {
    width: 360
    height: 360

    Loader {
        id: ldr
        anchors.fill: parent
    }
    
    property Component c: Rectangle {
        width: parent.width
        height: parent.height
        color: "pink"
    }
    
    MouseArea {
        anchors.fill: parent
        onClicked: {
            // toggle component on/off
            ldr.sourceComponent = ldr.sourceComponent ? null : c;
        }
    }
}