Fix crash when previewing a sprite sheet export when selecting an emp…

…ty layer group located at the bottom of the layer stack in the "Layers" combo box.

src/doc/sprite.cpp

@@ -311,7 +311,7 @@ LayerImage* Sprite::backgroundLayer() const
 Layer* Sprite::firstLayer() const
 {
   Layer* layer = root()->firstLayer();
-  while (layer->isGroup())
+  while (layer && layer->isGroup())
     layer = static_cast<LayerGroup*>(layer)->firstLayer();
   return layer;
 }