handle the case of invlide evidence id (can happen if we restart the server)

app/Feedback.php

@@ -36,9 +36,23 @@ class Feedback extends Model
         }
     }
 
+    private $report = null;
+
     public function report() : \Cylab\Mark\Evidence
     {
-        return Mark::get()->findEvidenceById($this->report_id);
+        if ($this->report != null) {
+            return $this->report;
+        }
+
+        try {
+            $this->report = Mark::get()->findEvidenceById($this->report_id);
+        } catch (\JsonRPC\Exception\ServerErrorException $ex) {
+            // may happen if the saved id is invalid (old data)
+            // should be replace by a static call Evidence::getNullReport()
+            $this->report = new \Cylab\Mark\Evidence(["id" => "?", "label" => "?", "time" => 0, "subject" => [], "score" => 0, "report" => "", "references" => [], "requests" => [], "profile" => ["label" => ""]]);
+        }
+
+        return $this->report;
     }
 
     public static function findByReportId(string $id)