Customizer does not show active Simple Sidebar in the widgets pane

[nickcernis]

When a custom widget area is assigned to a sidebar for a page, the Customizer still shows the default widget area instead of the custom one:

To reproduce:

1. Add a sidebar at Genesis → Simple Sidebars.
2. Add a widget to that sidebar area.
3. Edit a page and assign that sidebar to the Primary Sidebar area.
4. View that page, activate the Customizer, and visit the Widgets pane.

You'll see the Primary Sidebar instead of your custom one.

The impact of this is that custom sidebars can no longer be edited from the Customizer as of 2.1.0.

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Originally reported at https://wordpress.org/support/topic/no-longer-showing-the-sidebar-assigned-to-the-page-in-customizer/.