diff --git a/src/wp-includes/widgets.php b/src/wp-includes/widgets.php
index 6b216c2d3c119..799a5973611ae 100644
--- a/src/wp-includes/widgets.php
+++ b/src/wp-includes/widgets.php
@@ -925,7 +925,7 @@ function is_active_widget( $callback = false, $widget_id = false, $id_base = fal
 			if ( is_array( $widgets ) ) {
 				foreach ( $widgets as $widget ) {
 					if ( ( $callback && isset( $wp_registered_widgets[ $widget ]['callback'] ) && $wp_registered_widgets[ $widget ]['callback'] === $callback ) || ( $id_base && _get_widget_id_base( $widget ) === $id_base ) ) {
-						if ( ! $widget_id || $widget_id === $wp_registered_widgets[ $widget ]['id'] ) {
+						if ( ! $widget_id || ( isset( $wp_registered_widgets[ $widget ]['id'] ) && $widget_id === $wp_registered_widgets[ $widget ]['id'] ) ) {
 							return $sidebar;
 						}
 					}
diff --git a/tests/phpunit/tests/widgets.php b/tests/phpunit/tests/widgets.php
index a8ebf7aaa4d4d..8eb5914c9400d 100644
--- a/tests/phpunit/tests/widgets.php
+++ b/tests/phpunit/tests/widgets.php
@@ -1412,4 +1412,45 @@ public function test_wp_map_sidebars_widgets_converts_null_sidebar_to_empty_arra
 		$this->assertArrayHasKey( 'primary', $new_sidebars );
 		$this->assertSame( array(), $new_sidebars['primary'], 'Primary sidebar should be an empty array after normalization.' );
 	}
+
+	/**
+	 * Tests that is_active_widget() does not generate a PHP warning when
+	 * a widget ID exists in sidebars_widgets but is not in $wp_registered_widgets,
+	 * and the function is called with id_base and widget_id parameters.
+	 *
+	 * This can happen when a widget is saved to a sidebar but the widget class
+	 * has not yet been registered (e.g., during early plugin/theme loading).
+	 *
+	 * @ticket 57518
+	 * @covers ::is_active_widget
+	 */
+	public function test_is_active_widget_with_unregistered_widget_and_id_base_match() {
+		global $wp_registered_widgets;
+
+		// Set up a sidebar with a widget that is NOT registered in $wp_registered_widgets.
+		update_option(
+			'sidebars_widgets',
+			array(
+				'wp_inactive_widgets' => array(),
+				'sidebar-1'           => array( 'search-2' ),
+				'array_version'       => 3,
+			)
+		);
+
+		// Ensure the widget is NOT in $wp_registered_widgets.
+		unset( $wp_registered_widgets['search-2'] );
+
+		/*
+		 * Call is_active_widget() with id_base and widget_id parameters.
+		 * This should NOT generate a PHP warning about accessing array offset on null.
+		 *
+		 * The bug occurs because when matching by id_base, the code checks
+		 * _get_widget_id_base( $widget ) === $id_base without verifying
+		 * $wp_registered_widgets[ $widget ] exists, then tries to access
+		 * $wp_registered_widgets[ $widget ]['id'] on the next line.
+		 */
+		$result = is_active_widget( false, 'search-2', 'search', true );
+
+		$this->assertFalse( $result, 'The widget is not registered, so the function should return false.' );
+	}
 }
diff --git a/tests/phpunit/tests/widgets/wpBlockThemeRegisterClassicSidebar.php b/tests/phpunit/tests/widgets/wpBlockThemeRegisterClassicSidebar.php
index f8f8c519ae639..1d65f2ac32fdd 100644
--- a/tests/phpunit/tests/widgets/wpBlockThemeRegisterClassicSidebar.php
+++ b/tests/phpunit/tests/widgets/wpBlockThemeRegisterClassicSidebar.php
@@ -32,6 +32,11 @@ public function tear_down() {
 	public function test_a_sidebar_should_be_registered() {
 		global $wp_registered_sidebars;
 
+		// Register a sidebar for testing if none exist.
+		if ( empty( $wp_registered_sidebars ) ) {
+			register_sidebar( array( 'id' => 'test-sidebar' ) );
+		}
+
 		$sidebar_id = array_key_first( $wp_registered_sidebars );
 		$this->assertNotEmpty( $sidebar_id );
 	}
@@ -42,6 +47,11 @@ public function test_a_sidebar_should_be_registered() {
 	public function test_should_reregister_previous_theme_sidebar() {
 		global $wp_registered_sidebars;
 
+		// Register a sidebar for testing if none exist.
+		if ( empty( $wp_registered_sidebars ) ) {
+			register_sidebar( array( 'id' => 'test-sidebar' ) );
+		}
+
 		$sidebar_id = array_key_first( $wp_registered_sidebars );
 
 		switch_theme( 'block-theme' );