I don't know why the viewports are deleted with deleteLater(), but it leads
to an OpenGL context getting deselected behind our back, and so we don't know
when is a good time to re-select it. This leads to termination of the engine
when selecting File->Close (Ctrl+W) in the editor, because the graphics
re-initialization fails with no GL context active.
Instead, just delete the viewport widget immediately, which works fine at
least on Linux. This is also recommended by the Qt documentation at
http://doc.qt.io/qt-5/qopenglwidget.html.
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