[LyX/master] Fix crash in selection manager

[Richard Kimberly Heck]

On 11/19/19 7:33 AM, Juergen Spitzmueller wrote:
> commit 5d45c10fc785961d7c5f9e1cc291f14a116b1227
> Author: Juergen Spitzmueller <spitz at lyx.org>
> Date:   Tue Nov 19 13:47:32 2019 +0100
>
>     Fix crash in selection manager
>     
>     Patch by Patrick De Visschere

Fine for stable if needed.

Riki