[LyX/master] Fix nullptr warning

[André Pönitz]

On Tue, Mar 03, 2020 at 05:47:20AM +0100, Richard Kimberly Heck wrote:
> commit 84e91772eb255076384c96f662b56a0fbdf635a4
> Author: Richard Kimberly Heck <rikiheck at lyx.org>
> Date:   Tue Mar 3 00:08:46 2020 -0500
> 
>     Fix nullptr warning
> ---
>  src/frontends/qt/GuiToc.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/src/frontends/qt/GuiToc.h b/src/frontends/qt/GuiToc.h
> index f29a9af..48f68b3 100644
> --- a/src/frontends/qt/GuiToc.h
> +++ b/src/frontends/qt/GuiToc.h
> @@ -32,7 +32,7 @@ public:
>  	GuiToc(
>  		GuiView & parent, ///< the main window where to dock.
>  		Qt::DockWidgetArea area = Qt::LeftDockWidgetArea, ///< Position of the dock (and also drawer)
> -		Qt::WindowFlags flags = 0);
> +		Qt::WindowFlags flags = nullptr);

The initialization of QFlags with nullptr will be removed in Qt 6. 

Both 

	Qt::WindowFlags flags = nullptr);

and

	Qt::WindowFlags flags = {});

would work.

Andre'