fix potential fatal error on Interactivity API processing directives when wrong HTML tags

(cherry picked from commit 4d0212d)

Author: hugosolar

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

@@ -520,7 +520,12 @@ 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 ( ! is_countable( $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.