test_error_only_for_current_tab failed on mako #76

This test is failing on mako image #76:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/webbrowser_app/tests/test_tabs.py", line 166, in test_error_only_for_current_tab
    self.ensure_activity_view_visible()
  File "/usr/lib/python3/dist-packages/webbrowser_app/tests/__init__.py", line 165, in ensure_activity_view_visible
    self.main_window.open_toolbar().click_button("activityButton")
  File "/usr/lib/python3/dist-packages/autopilot/logging.py", line 46, in inner
    return f(instance, *args, **kwargs)
  File "/home/phablet/autopilot/ubuntuuitoolkit/_custom_proxy_objects/_toolbar.py", line 88, in click_button
    'Toolbar must be opened before calling click_button().')
ubuntuuitoolkit._custom_proxy_objects._common.ToolkitException: Toolbar must be opened before calling click_button().

I was unable to reproduce this despite trying many times. This bug is primarily to track this failure in CI.