"what is the height of the cube" fails sometimes

## Type of Issue

Select the type of issue:
- [x] Bug report (to report a bug)
- [ ] Feature request (to request an additional feature)
- [ ] Tracker (I am just using this as a tracker)
- [ ] Refactor request
- [ ] Documentation Ask

## Description

```
2021-02-10 11:53:59,559 [loco_mc_agent.py:252 - controller_step() INFO]: Incoming chats: [' what is the height of the cube']
2021-02-10 11:53:59,560 [loco_mc_agent.py:262 - controller_step() INFO]: Incoming chat: ['4ae1428d4b38d0fec98b99dfa8f37cd42ce4c279cec3ae69c0473b6683a39885:5103d4067a2144aaa0d363413041acc1' -> what is the height of the cube]
2021-02-10 11:54:01,736 [dialogue_manager.py:87 - step() INFO]: Dialogue stack pre-run_model: []
2021-02-10 11:54:01,738 [nsp_dialogue_manager.py:224 - get_logical_form() INFO]: Querying the semantic parsing model
2021-02-10 11:54:02,210 [nsp_dialogue_manager.py:231 - get_logical_form() INFO]: chat before lemmatization "what is the height of the cube"
2021-02-10 11:54:02,248 [nsp_dialogue_manager.py:234 - get_logical_form() INFO]: chat after lemmatization "what be the height of the cube"
2021-02-10 11:54:02,249 [nsp_dialogue_manager.py:238 - get_logical_form() INFO]: ttad pre-coref "what be the height of the cube" -> {'dialogue_type': 'GET_MEMORY', 'filters': {'triples': [{'pred_text': 'has_name', 'obj_text': 'cube'}], 'output': {'attribute': 'HEIGHT'}}}
2021-02-10 11:54:02,251 [nsp_dialogue_manager.py:248 - get_logical_form() INFO]: logical form before grammar update "{'dialogue_type': 'GET_MEMORY', 'filters': {'triples': [{'pred_text': 'has_name', 'obj_text': 'cube'}], 'output': {'attribute': 'HEIGHT'}}}
2021-02-10 11:54:02,251 [nsp_dialogue_manager.py:249 - get_logical_form() INFO]: logical form after grammar fix "{'dialogue_type': 'GET_MEMORY', 'filters': {'triples': [{'pred_text': 'has_name', 'obj_text': 'cube'}], 'output': {'attribute': 'HEIGHT'}}}"
2021-02-10 11:54:02,252 [nsp_dialogue_manager.py:169 - handle_logical_form() INFO]: logical form post-coref "9b3c733248c52c9491ae942d3bef072dc341bbc999a413e3b59c37ca817d16dc:cd3312b04166463497d819f30dc5acd6" -> {'dialogue_type': 'GET_MEMORY', 'filters': {'triples': [{'pred_text': 'has_name', 'obj_text': 'cube'}], 'output': {'attribute': 'HEIGHT'}}}
2021-02-10 11:54:02,258 [nsp_dialogue_manager.py:185 - handle_logical_form() INFO]: this model out: {'dialogue_type': 'GET_MEMORY', 'filters': {'triples': [{'pred_text': 'has_name', 'obj_text': 'cube'}], 'output': {'attribute': 'HEIGHT'}}}
2021-02-10 11:54:02,266 [get_memory_handler.py:172 - do_answer() ERROR]: list index out of range
Traceback (most recent call last):
File "/private/home/yuxuans/Workspace/tmp/droidlet/craftassist/agent/../../base_agent/dialogue_objects/get_memory_handler.py", line 166, in do_answer
return str(vals[0]), None
IndexError: list index out of range
2021-02-10 11:54:02,275 [craftassist_agent.py:242 - send_chat() INFO]: Sending chat: I don't understand what you're asking
2021-02-10 11:54:02,588 [loco_mc_agent.py:252 - controller_step() INFO]: Incoming chats: []
```