Added pytype None checks to station_server.py.

PiperOrigin-RevId: 608611221

openhtf/output/servers/station_server.py

@@ -456,7 +456,8 @@ def get(self):
     for file_name in os.listdir(self.history_path):
       if not file_name.endswith('.pb'):
         continue
-
+      if self.history_path is None:
+        raise ValueError('history_path is None, try calling initialize() first')
       if not os.path.isfile(os.path.join(self.history_path, file_name)):
         continue