fix potential fatal error on Interactivity API processing directives …

src/wp-includes/interactivity-api/class-wp-interactivity-api.php

@@ -439,7 +439,12 @@ private function _process_directives( string $html ) {
 					array_pop( $tag_stack );
 				}
 			} else {
-				if ( 0 !== count( $p->get_attribute_names_with_prefix( 'data-wp-each-child' ) ) ) {
+				$each_child_attrs = $p->get_attribute_names_with_prefix( 'data-wp-each-child' );
+				if ( null === $each_child_attrs ) {
+					continue;
+				}
+
+				if ( 0 !== count( $each_child_attrs ) ) {
 					/*
 					 * If the tag has a `data-wp-each-child` directive, jump to its closer
 					 * tag because those tags have already been processed.

tests/phpunit/tests/interactivity-api/wpInteractivityAPI.php

@@ -833,6 +833,31 @@ public static function data_html_with_unbalanced_tags() {
 		);
 	}
 
+	/**
+	 * Tests that the `process_directives` handles self-closing BR tags without
+	 * causing fatal errors and processes directives correctly.
+	 *
+	 * @ticket 63891
+	 * @covers ::process_directives
+	 */
+	public function test_process_directives_handles_br_self_closing_tags_with_invalid_closers() {
+		$this->interactivity->state(
+			'myPlugin',
+			array(
+				'id' => 'some-id',
+			),
+		);
+
+		$html = '</br><div data-wp-bind--id="myPlugin::state.id">Content</div>';
+
+		$processed_html = $this->interactivity->process_directives( $html );
+
+		$p = new WP_HTML_Tag_Processor( $processed_html );
+		$p->next_tag( 'div' );
+
+		$this->assertSame( 'some-id', $p->get_attribute( 'id' ) );
+	}
+
 	/**
 	 * Tests that the `process_directives` process the HTML outside a SVG tag.
 	 *