Fix of #3632: Crash if a thin wall object is not printable, therefore

no extrusion is generated.

Now the situation is detected and an exception is thrown.

src/libslic3r/Print.cpp

@@ -1591,6 +1591,8 @@ void Print::process()
         } else if (! this->config().complete_objects.value) {
         	// Initialize the tool ordering, so it could be used by the G-code preview slider for planning tool changes and filament switches.
         	m_tool_ordering = ToolOrdering(*this, -1, false);
+            if (m_tool_ordering.empty() || m_tool_ordering.last_extruder() == unsigned(-1))
+                throw std::runtime_error("The print is empty. The model is not printable with current print settings.");
         }
         this->set_done(psWipeTower);
     }