Return None for history source if media no longer exists (#1269)

Author: JonnyWong16

plexapi/base.py

@@ -953,8 +953,10 @@ def _reload(self, **kwargs):
         raise NotImplementedError('History objects cannot be reloaded. Use source() to get the source media item.')
 
     def source(self):
-        """ Return the source media object for the history entry. """
-        return self.fetchItem(self._details_key)
+        """ Return the source media object for the history entry
+            or None if the media no longer exists on the server.
+        """
+        return self.fetchItem(self._details_key) if self._details_key else None
 
     def delete(self):
         """ Delete the history entry. """