[Reg 6.6 -> 6.7] Q3DSurface and Q3DScatter crash in removeSeries

When removing a series from Q3DSurface and Q3DScatter in my unit tests the program crashes.

The reason: 

QQuickGraphsScatter does not do null pointer checks when doing this:

deleteDataItem(graphModel->instancingRootItem);
deleteDataItem(graphModel->selectionIndicator); 

QQuickGraphsSurface does not do null pointer checks when doing this:

m_model[i]->model->deleteLater();
m_model[i]->gridModel->deleteLater();
m_model[i]->proxyModel->deleteLater(); 

Before 6.7 RC everything worked well.