Bokeh: After the layout has been added to the root, exporting the png of a fig is not possible anymore

export_test.py

# -*- coding: utf-8 -*-
'''
After the layout has been added to the root, exporting the png of a fig is
not possible anymore

With "bokeh serve export_test.py", the console output for me is the following:
doc_layout works
fig works
Layout is added to root
doc_layout works
fig throws error
'''
from bokeh.io import curdoc
from bokeh.models import ColumnDataSource
from bokeh.plotting import figure
from bokeh.layouts import layout, widgetbox
from bokeh.io import export_png
from bokeh.models.widgets import Button


def download():
    try:
        export_png(doc_layout, filename="export doc_layout.png")
        print('doc_layout works')
    except Exception as ex:
        print('doc_layout throws error:', ex)
    try:
        export_png(fig, filename="export fig.png")
        print('fig works')
    except Exception as ex:
        print('fig throws error:', ex)


source = ColumnDataSource({'x': [2, 3, 4], 'y': [0.5, 1.5, 2.5]})
fig = figure(plot_width=1000, plot_height=300)
fig.hbar(y='y', right='x', height=0.4, source=source)

button = Button(label="Download")
button.on_click(download)
doc_layout = layout([[widgetbox(button)], [fig]], sizing_mode='fixed')

download()  # Test 1

print('Layout is added to root')
curdoc().add_root(doc_layout)
curdoc().title = "Export PNG test"

download()  # Test 2