Small bugfix

python/dune/perftool/pdelab/driver/error.py

@@ -147,8 +147,10 @@ def treepath_to_index(element, treepath, offset=0):
 def accumulate_L2_squared():
     element = get_trial_element()
     if isinstance(element, MixedElement):
-        tree_pathes = list(map(int, get_option("l2error_tree_path").split(',')))
-        assert len(tree_pathes) == element.value_size()
+        tree_pathes = (True,) * element.value_size()
+        if get_option("l2error_tree_path") is not None:
+            tree_pathes = list(map(int, get_option("l2error_tree_path").split(',')))
+            assert len(tree_pathes) == element.value_size()
         for i, path in enumerate(tree_pathes):
             if path:
                 _accumulate_L2_squared(get_treepath(element, i))