The crash is caused by the line seriesList: myGraph.pieSeries in main.qml
More precisely, the debugguer says me that the problem happens in the QGraphsView::insertSeries (internal method) because the QGraphsView object is nullptr.
So far, here is my understanding. All the "Missing QML.Element" messages are displayed when I add the line import QtGraphs in main.qml. Because it seems that nothing is loaded correctly, the GraphsView is not created which finally provokes the crash.
So I guess that if I fix the "Missing QML.Element" messages, everything will work fine.