fix null

examcloud-core-examwork-api-provider/src/main/java/cn/com/qmth/examcloud/core/examwork/api/provider/ExamCloudServiceProvider.java

@@ -181,6 +181,10 @@ public class ExamCloudServiceProvider extends ControllerSupport implements ExamC
         examInfo.setExamLimit(req.getExamLimit());
 
         Map<String, String> properties = req.getProperties();
+        if(properties == null){
+            properties = new HashMap<>();
+        }
+
         if(properties.get(ExamProperties.FREEZE_TIME.name())==null) {
         	properties.put(ExamProperties.FREEZE_TIME.name(),"0");
         }