`SplitEditorAreaPane.destroy` creates new widgets

[corranwebster]

Environment

Toolkit: Qt

Description

In the Qt backend, when SplitEditorAreaPane.destroy() is called it removes all open editors (this is good), but as a side-effect of this any DraggableTabWidgets in the layout create their empty_tab

Steps to Reproduce

Create and populate a SplitEditorAreaPane and then call destroy() to delete it. This is done in the tests for SplitEditorAreaPane for example.