MDL-80337 availability: more robust checking for plugin class presence.

Co-authored-by: Dustin Brisebois <dustin.brisebois@lambdasolutions.net>

availability/classes/frontend.php

@@ -117,6 +117,11 @@ public static function include_all_javascript($course, \cm_info $cm = null,
         foreach ($enabled as $plugin => $info) {
             // Create plugin front-end object.
             $class = '\availability_' . $plugin . '\frontend';
+            if (!class_exists($class)) {
+                continue;
+            }
+
+            /** @var \core_availability\frontend $frontend */
             $frontend = new $class();
 
             // Add to array of required YUI modules.

availability/classes/info.php

@@ -792,11 +792,10 @@ public static function completion_value_used($course, $cmid) {
         // plugin could also rely on the completion plugin.
         $pluginmanager = \core_plugin_manager::instance();
         $enabled = $pluginmanager->get_enabled_plugins('availability');
-        $componentparams = new \stdClass();
         foreach ($enabled as $plugin => $info) {
-            // Use the static method.
+            /** @var \core_availability\condition $class */
             $class = '\availability_' . $plugin . '\condition';
-            if ($class::completion_value_used($course, $cmid)) {
+            if (class_exists($class) && $class::completion_value_used($course, $cmid)) {
                 return true;
             }
         }

availability/classes/tree.php

@@ -220,9 +220,9 @@ public function __construct($structure, $lax = false, $root = true) {
 
             // First see if it's a condition. These have a defined type.
             if (isset($child->type)) {
-                // Look for a plugin of this type.
+                /** @var \core_availability\condition $classname */
                 $classname = '\availability_' . $child->type . '\condition';
-                if (!array_key_exists($child->type, $enabled)) {
+                if (!array_key_exists($child->type, $enabled) || !class_exists($classname)) {
                     if ($lax) {
                         // On load of existing settings, ignore if class
                         // doesn't exist.