Fix crash accessing empty list of sprites updating ContextBar fields (f…

…ix #4407)

Not sure if this will be the final solution for this crash, as a Doc
shouldn't have an empty list of sprites.

src/app/ui/context_bar.cpp

@@ -1770,7 +1770,9 @@ class ContextBar::SliceFields : public HBox {
  }
 
  void updateLayout() {
- const bool visible = (m_doc && !m_doc->sprite()->slices().empty());
+ const bool visible = (m_doc &&
+ m_doc->sprite() &&
+ !m_doc->sprite()->slices().empty());
  const bool relayout = (visible != m_combobox.isVisible() ||
  visible != m_action.isVisible());