Fix for rare error if vertex colors were somehow not active

src/builder.py

@@ -133,6 +133,7 @@ class ASEBuilder(object):
 
                 # Vertex Colors
                 if len(mesh_data.vertex_colors) > 0:
+                    if mesh_data.vertex_colors.active is not None:
                         vertex_colors = mesh_data.vertex_colors.active.data
                         for color in map(lambda x: x.color, vertex_colors):
                             geometry_object.vertex_colors.append(tuple(color[0:3]))