Double click in QHelpIndexWidget causes a segmentation fault

A double-click in the list of items displayed by the QHelpIndexWidget causes a segmentation fault. The bug manifests on both Linux and macOS, it has not been tested on Windows

This problem did not occur on either Windows, Linux or macOS with the Qt 5.12.3 build of the same code.

Steps to reproduce:
The attached minimal test program creates a help window. This window is split into two panes, the left-hand pane contains two tabs namely "Contents" and "Index".

Select the left-hand tab labelled "Contents", click on an item in the list, the contents of the right-hand pane will change to reflect the selected item.
Double-clicking on an item in the "Contents" tab will have the same effect.

Select the right-hand tab labelled "Index", click on an item in the list, the right-hand pane contents will change to reflect the selected item.
Double-click on an item in the "Index" tab, the application will close with a segmentation fault.