add null check #104

webmagic-core/src/main/java/us/codecraft/webmagic/Spider.java

@@ -187,10 +187,12 @@ public class Spider implements Runnable, Task {
         checkIfRunning();
         Scheduler oldScheduler = this.scheduler;
         this.scheduler = scheduler;
+        if (oldScheduler != null) {
             Request request;
             while ((request = oldScheduler.poll(this)) != null) {
                 this.scheduler.push(request, this);
             }
+        }
         return this;
     }