input: Write a python function to retrieve the default grids for the Hippo client and log the results.
  assert get_hippo_default_grids("/path/to/hippo") == "/path/to/hippo/app_settings/default_grids.xml"
  assert get_hippo_default_grids("/invalid/path") == None
output: def get_hippo_default_grids(base_path):
      import os
      default_grids_path = os.path.join(base_path, "app_settings", "default_grids.xml")
      if os.path.exists(default_grids_path):
          return default_grids_path
      else:
          return None