Vips modification feedback, re #203

controllers/sheets.php

@@ -1661,6 +1661,10 @@ class SheetsController extends StudipController
 
         vips_require_edit_permission($assignment);
 
+        if (!isset($assignment->options['feedback'])) {
+            $assignment->options['feedback'] = ['' => ''];
+        }
+
         $blocks = VipsBlock::findBySQL('course_id = ? ORDER BY name', [$assignment->course_id]);
 
         $this->assignment             = $assignment;