fix: more safety against empty layers (CURA-3290)

src/FffPolygonGenerator.cpp

@@ -524,11 +524,19 @@ void FffPolygonGenerator::removeEmptyFirstLayers(SliceDataStorage& storage, cons
         for (SliceMeshStorage& mesh : storage.meshes)
         {
             SliceLayer& layer = mesh.layers[layer_idx];
-            if (layer.parts.size() > 0 || (mesh.getSettingAsSurfaceMode("magic_mesh_surface_mode") != ESurfaceMode::NORMAL && layer.openPolyLines.size() > 0) )
+            if (mesh.getSettingAsSurfaceMode("magic_mesh_surface_mode") != ESurfaceMode::NORMAL && layer.openPolyLines.size() > 0)
             {
                 layer_is_empty = false;
                 break;
             }
+            for (const SliceLayerPart& part : layer.parts)
+            {
+                if (part.print_outline.size() > 0)
+                {
+                    layer_is_empty = false;
+                    break;
+                }
+            }
         }
         
         if (layer_is_empty) 

src/SkirtBrim.cpp

@@ -58,6 +58,10 @@ void SkirtBrim::getFirstLayerOutline(SliceDataStorage& storage, const unsigned i
     constexpr int smallest_line_length = 200;
     constexpr int largest_error_of_removed_point = 50;
     first_layer_outline.simplify(smallest_line_length, largest_error_of_removed_point); // simplify for faster processing of the brim lines
+    if (first_layer_outline.size() == 0)
+    {
+        logError("Couldn't generate skirt / brim! No polygons on first layer.");
+    }
 }
 
 int SkirtBrim::generatePrimarySkirtBrimLines(SliceDataStorage& storage, int start_distance, unsigned int primary_line_count, const int primary_extruder_skirt_brim_line_width, const int64_t primary_extruder_minimal_length, const Polygons& first_layer_outline, Polygons& skirt_brim_primary_extruder)