Guard for _active being None in __del__ method.

Lib/subprocess.py

@@ -618,7 +618,7 @@ class Popen(object):
             return
         # In case the child hasn't been waited on, check if it's done.
         self.poll(_deadstate=sys.maxint)
-        if self.returncode is None:
+        if self.returncode is None and _active is not None:
             # Child is still running, keep us alive until we can wait on it.
             _active.append(self)