QuickShow: Fix crash on startup

QSWidget::relayout get triggered before setting a bitmap to display
while setting the widget as the main widget of the window.

I've added a null check to the paint event too to make sure the
widget works with no bitmap set.

Applications/QuickShow/QSWidget.cpp

@@ -49,6 +49,9 @@ void QSWidget::set_bitmap(NonnullRefPtr<Gfx::Bitmap> bitmap)
 
 void QSWidget::relayout()
 {
+ if (m_bitmap.is_null())
+ return;
+
  Gfx::Size new_size;
  float scale_factor = (float)m_scale / 100.0f;
  new_size.set_width(m_bitmap->width() * scale_factor);
@@ -65,6 +68,9 @@ void QSWidget::resize_event(GUI::ResizeEvent& event)
 
 void QSWidget::paint_event(GUI::PaintEvent& event)
 {
+ if (m_bitmap.is_null())
+ return;
+
  GUI::Painter painter(*this);
  painter.add_clip_rect(event.rect());