Fix crash using yakuake

The hamburger menu seems to be null.

```
/home/apol/devel/frameworks/kconfigwidgets/src/khamburgermenu.h:97
menu=0x5580261dbe80) at
/home/apol/devel/frameworks/kconfigwidgets/src/khamburgermenu.cpp:109
Konsole::SessionController::showDisplayContextMenu (this=0x558025dfa2d0,
position=...) at
/home/apol/devel/frameworks/konsole/src/session/SessionController.cpp:1990
QtPrivate::FunctorCall<QtPrivate::IndexesList<0>, QtPrivate::List<QPoint
const&>, void, void (Konsole::SessionController::*)(QPoint
const&)>::call (f=(void
(Konsole::SessionController::*)(Konsole::SessionController * const,
const QPoint &)) 0x7f728c8a1920
<Konsole::SessionController::showDisplayContextMenu(QPoint const&)>,
o=0x558025dfa2d0, arg=0x7ffdb36086c0) at
/home/apol/devel/kde5/include/QtCore/qobjectdefs_impl.h:152
```