Fix more typing issues.

volatility/framework/interfaces/plugins.py

@@ -65,7 +65,7 @@ class PluginInterface(interfaces_configuration.ConfigurableInterface, validity.V
                  config_path: str,
                  progress_callback: validity.ProgressCallback = None) -> None:
         super().__init__(context, config_path)
-        self._progress_callback = progress_callback
+        self._progress_callback = progress_callback or (lambda f, s: None)
         # Plugins self validate on construction, it makes it more difficult to work with them, but then
         # the validation doesn't need to be repeated over and over again by externals
         if self.unsatisfied(context, config_path):

volatility/framework/symbols/wrappers.py

@@ -7,7 +7,7 @@ from volatility.framework import interfaces, validity
 class Flags(validity.ValidityRoutines):
     """Object that converts an integer into a set of flags based on their masks"""
 
-    def __init__(self, choices: typing.Mapping[str, int] = None) -> None:
+    def __init__(self, choices: typing.Mapping[str, int]) -> None:
         self._check_type(choices, collections.Mapping)
         for k, v in choices.items():
             self._check_type(k, str)