Add availability checks to widget related code (#15983)

This commit adds availability checks for the function get_current_screen and for the globals wp_widget_factory, wp_registered_widgets.

The function get_current_screen should only be called in admin pages, but according to the docs, we may have admin pages without the function so I think a check should be added. For example, I guess it may be possible to have an admin page with admin_footer but without the function defined.

Regarding the globals wp_widget_factory, wp_registered_widgets used in gutenberg_get_legacy_widget_settings function, the function is called as part of the filter block_editor_settings, I guess the filter may be executed in a situation where the widget globals are not initialized e.g: in a custom plugin page with a custom block editor. I think it is better to check the availability of the globals. If the globals are not available we will not pass available legacy widgets to the front end.

lib/widgets.php

@@ -11,6 +11,11 @@
  * @return boolean True if a screen containing the block editor is being loaded.
  */
 function gutenberg_is_block_editor() {
+	// If get_current_screen does not exist, we are neither in the standard block editor for posts, or the widget block editor.
+	// We can safely return false.
+	if ( ! function_exists( 'get_current_screen' ) ) {
+		return false;
+	}
 	$screen = get_current_screen();
 	return $screen->is_block_editor() || 'gutenberg_page_gutenberg-widgets' === $screen->id;
 }
@@ -100,33 +105,38 @@ function gutenberg_get_legacy_widget_settings() {
 
 	$has_permissions_to_manage_widgets = current_user_can( 'edit_theme_options' );
 	$available_legacy_widgets          = array();
-	global $wp_widget_factory, $wp_registered_widgets;
-	foreach ( $wp_widget_factory->widgets as $class => $widget_obj ) {
-		$available_legacy_widgets[ $class ] = array(
-			'name'             => html_entity_decode( $widget_obj->name ),
-			// wp_widget_description is not being used because its input parameter is a Widget Id.
-			// Widgets id's reference to a specific widget instance.
-			// Here we are iterating on all the available widget classes even if no widget instance exists for them.
-			'description'      => isset( $widget_obj->widget_options['description'] ) ?
-				html_entity_decode( $widget_obj->widget_options['description'] ) :
-				null,
-			'isCallbackWidget' => false,
-			'isHidden'         => in_array( $class, $core_widgets, true ),
-		);
+	global $wp_widget_factory;
+	if ( ! empty( $wp_widget_factory ) ) {
+		foreach ( $wp_widget_factory->widgets as $class => $widget_obj ) {
+			$available_legacy_widgets[ $class ] = array(
+				'name'             => html_entity_decode( $widget_obj->name ),
+				// wp_widget_description is not being used because its input parameter is a Widget Id.
+				// Widgets id's reference to a specific widget instance.
+				// Here we are iterating on all the available widget classes even if no widget instance exists for them.
+				'description'      => isset( $widget_obj->widget_options['description'] ) ?
+					html_entity_decode( $widget_obj->widget_options['description'] ) :
+					null,
+				'isCallbackWidget' => false,
+				'isHidden'         => in_array( $class, $core_widgets, true ),
+			);
+		}
 	}
-	foreach ( $wp_registered_widgets as $widget_id => $widget_obj ) {
-		if (
-			is_array( $widget_obj['callback'] ) &&
-			isset( $widget_obj['callback'][0] ) &&
-			( $widget_obj['callback'][0] instanceof WP_Widget )
-		) {
-			continue;
+	global $wp_registered_widgets;
+	if ( ! empty( $wp_registered_widgets ) ) {
+		foreach ( $wp_registered_widgets as $widget_id => $widget_obj ) {
+			if (
+				is_array( $widget_obj['callback'] ) &&
+				isset( $widget_obj['callback'][0] ) &&
+				( $widget_obj['callback'][0] instanceof WP_Widget )
+			) {
+				continue;
+			}
+			$available_legacy_widgets[ $widget_id ] = array(
+				'name'             => html_entity_decode( $widget_obj['name'] ),
+				'description'      => html_entity_decode( wp_widget_description( $widget_id ) ),
+				'isCallbackWidget' => true,
+			);
 		}
-		$available_legacy_widgets[ $widget_id ] = array(
-			'name'             => html_entity_decode( $widget_obj['name'] ),
-			'description'      => html_entity_decode( wp_widget_description( $widget_id ) ),
-			'isCallbackWidget' => true,
-		);
 	}
 
 	$settings['hasPermissionsToManageWidgets'] = $has_permissions_to_manage_widgets;