Bug fix - don't clear agents marked as None.

Author: adamjeffries

kaggle_environments/core.py

@@ -574,7 +574,8 @@ def act(none_action=None):
 
         def destroy():
             for a in agents:
-                a.destroy()
+                if a != None:
+                    a.destroy()
 
         return structify({"act": act, "destroy": destroy})