WP_Customize_Widgets::preview_sidebars_widgets() can return false

[ocean90]

Seeing this PHP notice on dotorg:

​E_WARNING: /wp-includes/class-wp-customize-widgets.php:385 - array_merge(): Argument #3 is not an array
array_merge(): Argument #3 is not an array
Timestamp: Mon, 25 Apr 2016 08:52:29 +0000 (1461574349)
File: /wp-includes/class-wp-customize-widgets.php
Line: 385

This seems to be caused by [37166] where the sidebars_widgets callback was moved before the wp_get_sidebars_widgets() call.
WP_Customize_Widgets::preview_sidebars_widgets() is using get_option( 'sidebars_widgets' ) without defining a default value so it falls back to false in case the site has no widgets.

[westonruter]

Related: #27965 (Error in theme customizer if all widget areas are empty)

[westonruter]

Added unit test in 36660.2.diff​ to verify the fix. Without the patch, the unit test fails with:

1) Tests_WP_Customize_Widgets::test_customize_register_with_deleted_sidebars
array_merge(): Argument #3 is not an array

src/wp-includes/class-wp-customize-widgets.php:385
tests/phpunit/tests/customize/widgets.php:112

[westonruter]

In 37352:

Customize: Handle filtering sidebars_widgets when the underlying option is non-existent.

Fixes warning related to a non-array argument for array_merge() in WP_Customize_Widgets::customize_register().

See [37166].
See #36389.
Fixes #36660.

[westonruter]

Re-opening for 4.5.2 consideration.

[ocean90]

In 37453:

Customize: Handle filtering sidebars_widgets when the underlying option is non-existent.

Fixes warning related to a non-array argument for array_merge() in WP_Customize_Widgets::customize_register().

Merge of [37352] to the 4.5 branch.

See [37166].
See #36389.
Fixes #36660.