`SplitEditorAreaPane.destroy` creates new widgets

## 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.