fix null checking and add test

Author: hugosolar

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

@@ -440,11 +440,8 @@ private function _process_directives( string $html ) {
 				}
 			} else {
 				$each_child_attrs = $p->get_attribute_names_with_prefix( 'data-wp-each-child' );
-				if ( ! is_countable( $each_child_attrs ) ) {
-					continue;
-				}
 
-				if ( 0 !== count( $each_child_attrs ) ) {
+				if ( null !== $each_child_attrs && 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

@@ -830,9 +830,37 @@ public static function data_html_with_unbalanced_tags() {
 			'SPAN opener inside' => array( '<div data-wp-bind--id="myPlugin::state.id"><span>Inner content</div>' ),
 			'SPAN closer after'  => array( '<div data-wp-bind--id="myPlugin::state.id">Inner content</div></span>' ),
 			'SPAN overlapping'   => array( '<div data-wp-bind--id="myPlugin::state.id"><span>Inner content</div></span>' ),
+			'BR self-closing'    => array( '<div data-wp-bind--id="myPlugin::state.id">Content<br></br></div>' ),
 		);
 	}
 
+	/**
+	 * Tests that the `process_directives` handles self-closing tags with invalid
+	 * closing tags without causing fatal errors.
+	 *
+	 * @covers ::process_directives
+	 *
+	 * @expectedIncorrectUsage WP_Interactivity_API::_process_directives
+	 */
+	public function test_process_directives_handles_self_closing_tags_with_invalid_closers() {
+		$this->interactivity->state(
+			'myPlugin',
+			array(
+				'id' => 'some-id',
+			),
+		);
+
+		$html = '<div data-wp-bind--id="myPlugin::state.id">Content<br></br></div>';
+
+		$processed_html = $this->interactivity->process_directives( $html );
+
+		$this->assertSame( $html, $processed_html );
+
+		$p = new WP_HTML_Tag_Processor( $processed_html );
+		$p->next_tag( 'div' );
+		$this->assertNull( $p->get_attribute( 'id' ) );
+	}
+
 	/**
 	 * Tests that the `process_directives` process the HTML outside a SVG tag.
 	 *