Fix TypeErrors: sequence item expected str, None found (ex: ','.join([None, 'a', 'b']))

taiga/projects/history/templatetags/functions.py

@@ -40,4 +40,12 @@ def verbose_name(obj_class, field_name):
 
 @library.global_function
 def lists_diff(list1, list2):
-    return (list(set(list1) - set(list2)))
+    """
+    Get the difference of two list and remove None values.
+
+    >>> list1 = ["a", None, "b", "c"]
+    >>> list2 = [None, "b", "d", "e"]
+    >>> list(filter(None.__ne__, set(list1) - set(list2)))
+    ['c', 'a']
+    """
+    return list(filter(None.__ne__, set(list1) - set(list2)))