diff --git a/src/wp-includes/class-wp-theme.php b/src/wp-includes/class-wp-theme.php
index ee74099766..7c25d07b2c 100644
--- a/src/wp-includes/class-wp-theme.php
+++ b/src/wp-includes/class-wp-theme.php
@@ -1576,6 +1576,11 @@ final class WP_Theme implements ArrayAccess {
 	 * @return bool
 	 */
 	public function is_block_theme() {
+		if ( ! did_action( 'setup_theme' ) ) {
+			_doing_it_wrong( __METHOD__, __( 'This method should not be called before themes are set up.' ), '6.8.0' );
+			return false;
+		}
+
 		if ( isset( $this->block_theme ) ) {
 			return $this->block_theme;
 		}
diff --git a/src/wp-includes/default-filters.php b/src/wp-includes/default-filters.php
index 36f3c949dd..2bc2e61e4a 100644
--- a/src/wp-includes/default-filters.php
+++ b/src/wp-includes/default-filters.php
@@ -737,6 +737,7 @@ add_action( 'save_post_wp_template_part', 'wp_set_unique_slug_on_create_template
 add_action( 'wp_enqueue_scripts', 'wp_enqueue_block_template_skip_link' );
 add_action( 'wp_footer', 'the_block_template_skip_link' ); // Retained for backwards-compatibility. Unhooked by wp_enqueue_block_template_skip_link().
 add_action( 'after_setup_theme', 'wp_enable_block_templates', 1 );
+add_action( 'after_setup_theme', 'wp_set_editor_default_mode', 2 ); // Run after enabling block templates.
 add_action( 'wp_loaded', '_add_template_loader_filters' );
 
 // wp_navigation post type.
diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index a9992071f0..4db40681bb 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -69,11 +69,6 @@ function create_initial_post_types() {
 		)
 	);
 
-	// Enhance page editor for block themes by rendering template and content blocks.
-	if ( wp_is_block_theme() && current_theme_supports( 'block-templates' ) ) {
-		add_post_type_support( 'page', 'editor', array( 'default-mode' => 'template-locked' ) );
-	}
-
 	register_post_type(
 		'attachment',
 		array(
@@ -8529,3 +8524,14 @@ function wp_create_initial_post_meta() {
 		)
 	);
 }
+
+/**
+ * Sets the default editor mode based on support for block templates.
+ *
+ * @since 6.8.0
+ */
+function wp_set_editor_default_mode() {
+	if ( wp_is_block_theme() && current_theme_supports( 'block-templates' ) ) {
+		add_post_type_support( 'page', 'editor', array( 'default-mode' => 'template-locked' ) );
+	}
+}