render template widget correctly, fixes #1115, re #1102

Merge request studip/studip!666

lib/classes/sidebar/ResourceTreeWidget.class.php

@@ -116,12 +116,7 @@ class ResourceTreeWidget extends SidebarWidget
         $variables['max_open_depth'] = 0;
         $variables['layout_css_classes'] = $this->layout_css_classes;
 
-        $template = $GLOBALS['template_factory']->open(
-            $this->template
-        );
-        $template->set_attributes($variables);
-        $template->set_layout('sidebar/widget-layout');
-        return $template->render();
+        return parent::render($variables);
     }
 
     public function setCurrentResource(Resource $resource)

lib/classes/sidebar/SidebarWidget.php

@@ -9,16 +9,13 @@
  */
 class SidebarWidget extends Widget
 {
-    protected $id = '';
     protected $additional_attributes = [];
 
-
     public function __construct()
     {
         $this->layout = 'sidebar/widget-layout.php';
     }
 
-
     /**
      * Sets the ID of the HTML element that represents the widget.
      *
@@ -30,7 +27,6 @@ class SidebarWidget extends Widget
         $this->id = $id;
     }
 
-
     /**
      * Returns the ID of this widget, if it is set.
      *
@@ -38,7 +34,7 @@ class SidebarWidget extends Widget
      */
     public function getId() : string
     {
-        return $this->id;
+        return $this->id ?? '';
     }
 
     /**
@@ -101,7 +97,6 @@ class SidebarWidget extends Widget
         unset($this->additional_attributes[$key]);
     }
 
-
     /**
      * Renders the widget.
      * The widget will only be rendered if it contains at least one element.