fixbugs

stmms-web/src/main/java/cn/com/qmth/stmms/admin/exam/ProblemHistoryController.java

@@ -95,7 +95,9 @@ public class ProblemHistoryController extends BaseExamController {
             student.setProblemType(problemMap.get(history.getProblemId()));
             student.setMarkTime(DateUtils.formatDateTime(history.getCreateTime()));
             student.setLibraryId(history.getLibraryId());
-            student.setMarkLogin(userService.findById(history.getUserId()).getLoginName());
+            if (history.getUserId() != null) {
+                student.setMarkLogin(userService.findById(history.getUserId()).getLoginName());
+            }
             list.add(student);
         }
         model.addAttribute("resultList", list);