Fix segfault when QtQuick isn't available

Summary:
For some reason, my Kdev doesn't find qtquick (even though its installed), but that's peripheral to the problem here.

This trivial patch fixes the issue.

KDevelop crashes with the (shortened) back trace:

#5 <signal handler called>
#6 QQuickItem::height (this=0x0) at items/qquickitem.cpp:6234
#7 0x00007f096fd062dd in AssistantPopup::updateLayout() (this=0x4d3c080) at /home/pedro.ferreira/src/kdev/kdevplatform/shell/assistantpopup.cpp:336
#8 0x00007f096fd06425 in AssistantPopup::eventFilter(QObject*, QEvent*) (this=0x4d3c080, object=<optimized out>, event=0x7ffe6fcc8b20) at /home/pedro.ferreira/src/kdev/kdevplatform/shell/assistantpopup.cpp:259
#9 0x000000300883dbdd in QCoreApplicationPrivate::sendThroughObjectEventFilters (this=this@entry=0x187d3f0, receiver=receiver@entry=0x90c23a0, event=event@entry=0x7ffe6fcc8b20) at kernel/qcoreapplication.cpp:1072
#10 0x000000300bb5ae7c in QApplicationPrivate::notify_helper (this=this@entry=0x187d3f0, receiver=receiver@entry=0x90c23a0, e=e@entry=0x7ffe6fcc8b20) at kernel/qapplication.cpp:3712
#11 0x000000300bb5ffb0 in QApplication::notify (this=0x7ffe6fccac50, receiver=0x90c23a0, e=0x7ffe6fcc8b20) at kernel/qapplication.cpp:3499
#12 0x000000300883d9bd in QCoreApplication::notifyInternal (this=0x7ffe6fccac50, receiver=receiver@entry=0x90c23a0, event=event@entry=0x7ffe6fcc8b20) at kernel/qcoreapplication.cpp:965
#13 0x000000300bb93bdc in QCoreApplication::sendEvent (event=0x7ffe6fcc8b20, receiver=0x90c23a0) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:224

Reviewers: kfunk, KDevelop

Reviewed By: kfunk, KDevelop

Subscribers: kfunk, kdevelop-devel

Projects: KDevelop

Differential Revision: https://phabricator.kde.org/D1101