Gracefully handle widget renames and disappearances

[westonruter]

Right now we're getting assertion warnings at https://github.com/x-team/wp-widget-customizer/blob/1b3f87e98631fda33f3dfb8966044a1aae64bdfd/widget-customizer.php#L261

[westonruter]

For example:

diff --git a/widget-customizer.php b/widget-customizer.php
index 2c6f8d0..f4d49da 100644
--- a/widget-customizer.php
+++ b/widget-customizer.php
@@ -258,7 +258,11 @@ class Widget_Customizer {
             * Add setting for each widget, and a control for each active widget (located in a sidebar)
             */
            foreach ( $sidebar_widget_ids as $i => $widget_id ) {
-               assert( isset( $GLOBALS['wp_registered_widgets'][$widget_id] ) );
+               // A widget must have been renamed in code, so we'll have to skip it
+               // @todo Do we also need to purge it from the widgets_sidebars?
+               if ( ! array_key_exists( $widget_id, $GLOBALS['wp_registered_widgets'] ) ) {
+                   continue;
+               }
                $registered_widget = $GLOBALS['wp_registered_widgets'][$widget_id];
                $setting_id = self::get_setting_id( $widget_id );
                $wp_customize->add_setting( $setting_id, self::get_setting_args( $setting_id ) );

[westonruter]

What does core do right now when it comes across a widget ID in sidebars_widgets which does not exist in registered_widgets?

If they save the sidebars_widgets will the widget ID be removed entirely, or will it just be moved to inactive_widgets and disabled until the widget reappears (if ever)?

[westonruter]

dynamic_sidebar() does this:

    foreach ( (array) $sidebars_widgets[$index] as $id ) {
        if ( !isset($wp_registered_widgets[$id]) ) continue;