After setting the SSTATE_MIRRORS in local.conf according to the example,
settings button not available showing below stack trace:

Traceback (most recent call last):
  File "/poky/bitbake/lib/bb/ui/crumbs/imageconfigurationpage.py", line 496, in settings_button_clicked_cb
    response, settings_changed = self.builder.show_simple_settings_dialog()
  File "/poky/bitbake/lib/bb/ui/crumbs/builder.py", line 1258, in show_simple_settings_dialog
    handler = self.handler)
  File "/poky/bitbake/lib/bb/ui/crumbs/hig/simplesettingsdialog.py", line 98, in __init__
    self.create_visual_elements()
  File "/poky/bitbake/lib/bb/ui/crumbs/hig/simplesettingsdialog.py", line 888, in create_visual_elements
    self.nb.append_page(self.create_shared_state_page(), gtk.Label("Shared state"))
  File "/poky/bitbake/lib/bb/ui/crumbs/hig/simplesettingsdialog.py", line 335, in create_shared_state_page
    sm_list = ["Custom", sstatemirror_fields[1], sstatemirror_fields[0]]
IndexError: list index out of range

local.conf set SSTATE_MIRRORS as below:

SSTATE_MIRRORS ?= "\
file://.* http://someserver.tld/share/sstate/PATH;downloadfilename=PATH \n \
file://.* file:///some/local/dir/sstate/PATH"

Settings button becomes not available, because when create_shared_state_page() in simplesettingsdialog.py,
searched_string was set to "file://" as below:
searched_string = "file://"

The logic to parse each sstate mirrors regex and url is not right, if SSTATE_MIRRORS has this pattern:
file://.* file:///some/local/dir/sstate/PATH