3111 Handle the supervisor deleting the seminar document as the authors are trying to replace it

core/src/main/java/se/su/dsv/scipro/finalseminar/FinalSeminarUploadControllerImpl.java

@@ -56,7 +56,10 @@ public class FinalSeminarUploadControllerImpl implements FinalSeminarUploadContr
     @Override
     @Transactional
     public SeminarDocumentUpload replaceSeminarDocument(final FinalSeminar finalSeminar, final FileUpload fileUpload) {
-        fileDescriptionService.delete(finalSeminar.getDocument());
+        if (finalSeminar.getDocument() != null) {
+            // if someone else deletes the document while it is being replaced
+            fileDescriptionService.delete(finalSeminar.getDocument());
+        }
         return storeAndSetSeminarDocument(finalSeminar, fileUpload);
     }