Fix crash on scrolling through the doc attached to bug 274905
The root-area can be NULL.

[KCrash Handler]
6 0x00007fb31e1344fa in KoTextLayoutRootArea::setPage (this=0x0, textpage=0x1901780) at /home/snoopy/src/calligra/libs/textlayout/KoTextLayoutRootArea.cpp:79
7 0x00007fb3104c98bf in KWCopyShape::paint (this=0xb2a360, painter=..., converter=...) at /home/snoopy/src/calligra/words/part/frames/KWCopyShape.cpp:92
8 0x00007fb324bad8ca in KoShapeManager::paintShape (this=0x1471640, shape=0xb2a360, painter=..., converter=..., forPrint=false) at /home/snoopy/src/calligra/libs/flake/KoShapeManager.cpp:369
9 0x00007fb324babdb7 in KoShapeManagerPaintingStrategy::paint (this=0x1471bb0, shape=0xb2a360, painter=..., converter=..., forPrint=false) at /home/snoopy/src/calligra/libs/flake/KoShapeManagerPaintingStrategy.cpp:52
10 0x00007fb324bad64a in KoShapeManager::paint (this=0x1471640, painter=..., converter=..., forPrint=false) at /home/snoopy/src/calligra/libs/flake/KoShapeManager.cpp:335
11 0x00007fb3104787c8 in KWCanvasBase::paint (this=0x1471108, painter=..., paintRect=...) at /home/snoopy/src/calligra/words/part/KWCanvasBase.cpp:316
12 0x00007fb31047e94c in KWCanvas::paintEvent (this=0x14710e0, ev=0x7fff99ccd5a0) at /home/snoopy/src/calligra/words/part/KWCanvas.cpp:156