Fix issues

* Fix issue where children nodes could be None
* Skip non-function special methods

Author: ktbarrett

sphinxcontrib/prettyspecialmethods.py

@@ -47,7 +47,7 @@ def xf(name_node, parameters_node):
                 "",
                 [
                     SphinxNodes.desc_parameter("", "self"),
-                    *parameters_node.children,
+                    *(parameters_node.children if parameters_node is not None else []),
                 ],
             ),
         )
@@ -204,6 +204,11 @@ def apply(self):
             if method_name in SPECIAL_METHODS:
                 parameters_node = ref.next_node(SphinxNodes.desc_parameterlist)
 
+                # Some special methods might not be functions (e.g. `__hash__ = None`).
+                # In this case, skip.
+                if parameters_node is None:
+                    continue
+
                 name_node.replace_self(
                     SPECIAL_METHODS[method_name](name_node, parameters_node)
                 )