Misplaced Joint{} element may crash application

A `Joint` element without skeletonRoot (or with an invalid one) causes the whole application to crash without explanation.

The problem occurs if a Joint is placed as it is inside a `view3D` (obviously, the Joint shouldn't do anything if there is no skeleton)

How to reproduce:

Open simpleSkinning example. Anywhere inside the main.qml view element, add a `Joint` element, for exemple line 64 :

    View3D {
        id: view
        Joint{}
        anchors.fill: parent
[...]

Application will crash on launch.

The same will happen for any Qt Quick 3D example containing a view3D.