fix: warn when lessons are misconfigured

user/plugins/m3-p2/m3-p2.php

@@ -223,7 +223,7 @@ class M3P2Plugin extends Plugin
         $uri = $this->grav['uri'];
 //        var_dump($uri); // best doc ftw
 
-        // GRAV's routing makes use hook 404 pages and do the URL match ourselves
+        // GRAV's routing makes us hook 404 pages and do the URL match ourselves
         if ("lessons" != $uri->paths(0)) {
             return; // perhaps not
         }
@@ -239,8 +239,6 @@ class M3P2Plugin extends Plugin
         /** @var Pages $pages */
         $pages = $this->grav['pages'];
 
-        // VERY inefficient : TODO: cache this
-//        $target_pages = $pages->all()->filter(function ($p) use ($lesson_slug) {
         $target_pages = array_filter($pages->instances(), function (Page $p) use ($lesson_slug) {
 
             $page_config = $p->header();
@@ -249,6 +247,15 @@ class M3P2Plugin extends Plugin
             }
 
             foreach ($page_config->lessons as $lesson) {
+
+                if ( ! isset($lesson['slug'])) {
+                    trigger_error(
+                        "Misconfigured lesson (missing a slug) in ".($p->filePathClean()),
+                        E_USER_WARNING
+                    );
+                    continue;
+                }
+
                 if ($lesson_slug == $lesson['slug']) {
                     return true; // yay, this page is part of the lesson
                 }