Streamlet map add_rows error

Hi,

I want to animate a map by periodically adding more rows to be displayed. Here is my toy example:

import streamlit as st
import numpy as np
import pandas as pd
import time


def make_data():
    return pd.DataFrame(np.random.randn(10, 2) + 10., columns=["lon", "lat"])


"# Animate Map"

data = make_data()

map_chart = st.map(data)
for i in range(10):
    more_data = make_data()
    map_chart.add_rows(data=more_data)
    time.sleep(2)

I get the following error:

AttributeError: 'NoneType' object has no attribute 'data'
Traceback:
File "/Users/kirill.trapeznikov/miniconda3/envs/sabi/lib/python3.7/site-packages/streamlit/script_runner.py", line 337, in _run_script
    exec(code, module.__dict__)
File "/Users/kirill.trapeznikov/code/str/mitch/demo/main.py", line 18, in <module>
    map_chart.add_rows(data=more_data)
File "/Users/kirill.trapeznikov/miniconda3/envs/sabi/lib/python3.7/site-packages/streamlit/delta_generator.py", line 552, in add_rows
    _enqueue_message(msg)
File "/Users/kirill.trapeznikov/miniconda3/envs/sabi/lib/python3.7/site-packages/streamlit/delta_generator.py", line 626, in _enqueue_message
    ctx.enqueue(msg)
File "/Users/kirill.trapeznikov/miniconda3/envs/sabi/lib/python3.7/site-packages/streamlit/report_thread.py", line 89, in enqueue
    self._enqueue(msg)
File "/Users/kirill.trapeznikov/miniconda3/envs/sabi/lib/python3.7/site-packages/streamlit/report_session.py", line 185, in enqueue
    self._report.enqueue(msg)
File "/Users/kirill.trapeznikov/miniconda3/envs/sabi/lib/python3.7/site-packages/streamlit/report.py", line 105, in enqueue
    self._master_queue.enqueue(msg)
File "/Users/kirill.trapeznikov/miniconda3/envs/sabi/lib/python3.7/site-packages/streamlit/report_queue.py", line 86, in enqueue
    composed_delta = compose_deltas(old_msg.delta, msg.delta)
File "/Users/kirill.trapeznikov/miniconda3/envs/sabi/lib/python3.7/site-packages/streamlit/report_queue.py", line 145, in compose_deltas
    data_frame.add_rows(composed_delta, new_delta, name=new_delta.add_rows.name)
File "/Users/kirill.trapeznikov/miniconda3/envs/sabi/lib/python3.7/site-packages/streamlit/elements/data_frame.py", line 431, in add_rows
    if len(df1.data.cols) == 0: