Fix some crashes when run under GCC6 UBSAN

Sample backtrace:

/usr/include/qt4/QtCore/qobject.h:378:5: runtime error:
reference binding to null pointer of type 'const struct
QAction'

6 0x00007f1ea6993738 in qobject_cast<QAction*> (object=0x0) at
/usr/include/qt4/QtCore/qobject.h:378
7 0x00007f1ea696dfe0 in QuickOpenWidget::updateProviders
(this=this@entry=0x55a637ddbe40) at
/home/kfunk/devel/src/kdevplatform/plugins/quickopen/quickopenplugin.cpp:464
8 0x00007f1ea6971ef0 in QuickOpenWidget::QuickOpenWidget
(this=<optimized out>, title=..., model=<optimized out>,
initialItems=..., initialScopes=..., listOnly=<optimized out>,
noSearchField=false) at
/home/kfunk/devel/src/kdevplatform/plugins/quickopen/quickopenplugin.cpp:349
9 0x00007f1ea69720d3 in QuickOpenWidgetDialog::QuickOpenWidgetDialog
(this=0x55a639eaeb10, title=..., model=<optimized out>,
initialItems=..., initialScopes=..., listOnly=<optimized out>,
noSearchField=false) at
/home/kfunk/devel/src/kdevplatform/plugins/quickopen/quickopenplugin.cpp:436