Crash occured when "Use Settings For" was set to Render and "Visible Objects Only" was ticked and a single parent object is in the scene but disabled for render.

Because the only object attached to the root of the project was disabled for export, there was no "null" root node added to the export graph. This change will always add an empty "null" node with no children to the graph at the start. Other objects will get added to its children as required.