Add fallback on last extruder of previous layer when used extruder li…

…st is empty

CURA-11649

src/FffGcodeWriter.cpp

@@ -1173,13 +1173,29 @@ FffGcodeWriter::ProcessLayerResult FffGcodeWriter::processLayer(const SliceDataS
     const std::vector<ExtruderUse>& extruder_order
         = (layer_nr < 0) ? extruder_order_per_layer_negative_layers[extruder_order_per_layer_negative_layers.size() + layer_nr] : extruder_order_per_layer[layer_nr];
 
-    const coord_t first_outer_wall_line_width = scene.extruders[extruder_order.front().extruder_nr].settings_.get<coord_t>("wall_line_width_0");
+    size_t first_extruder;
+    if (extruder_order.size() > 0)
+    {
+        first_extruder = extruder_order.front().extruder_nr;
+    }
+    else
+    {
+        // find the last extruder used in the previous layer
+        size_t last_extruder_nr_layer = layer_nr - 1;
+        while (extruder_order_per_layer[last_extruder_nr_layer].size() == 0 && last_extruder_nr_layer >= 0)
+        {
+            last_extruder_nr_layer--;
+        }
+        first_extruder = extruder_order_per_layer[last_extruder_nr_layer].back().extruder_nr;
+    }
+
+    const coord_t first_outer_wall_line_width = scene.extruders[first_extruder].settings_.get<coord_t>("wall_line_width_0");
     LayerPlan& gcode_layer = *new LayerPlan(
         storage,
         layer_nr,
         z,
         layer_thickness,
-        extruder_order.front().extruder_nr,
+        first_extruder,
         fan_speed_layer_time_settings_per_extruder,
         comb_offset_from_outlines,
         first_outer_wall_line_width,