Crash in updating SliceFields in ContextBar / a Doc has an empty list of sprites

[sentry-io]

Sentry Issue: ASEPRITE-237

EXCEPTION_ACCESS_VIOLATION_READ / 0x1ae000001a8: Fatal Error: EXCEPTION_ACCESS_VIOLATION_READ / 0x1ae000001a8
  File "sprites.h", line 35, in doc::Sprites::front
    Sprite* front() const { return m_sprites.front(); }
  File "document.h", line 27, in doc::Document::sprite
    Sprite* sprite() { return m_sprites.empty() ? NULL: m_sprites.front(); }
  File "context_bar.cpp", line 1773, in app::ContextBar::SliceFields::updateLayout
    const bool visible = (m_doc && !m_doc->sprite()->slices().empty());
  File "context_bar.cpp", line 1728, in app::ContextBar::SliceFields::selectSlices
    updateLayout();
  File "context_bar.cpp", line 2280, in app::ContextBar::updateForTool
    updateSliceFields(UIContext::instance()->activeSite());
...
(36 additional frame(s) were not displayed)

[dacap]

Regression https://igara-studio.sentry.io/share/issue/3c98fc17c9d646be99b73cf328a1b21b/