source is null for getProperty(null, "2")

- [x] 我已经在 [issues](https://github.com/alibaba/arthas/issues) 里搜索，没有重复的issue。

### 环境信息

* `arthas-boot.jar` 或者 `as.sh` 的版本： 4.1.2

```
-12-17 16:06:30 [syncDataThreadPool5-1] WARN c.t.a.c.c.m.TimeTunnelAdviceListener -tt failed.
com.taobao.arthas.core.command.express.ExpressException: ognl.OgnlException: source is null for getProperty(null, "2")
at com.taobao.arthas.core.command.express.OgnlExpress.get(OgnlExpress.java:39)
at com.taobao.arthas.core.command.express.OgnlExpress.is(OgnlExpress.java:45)
at com.taobao.arthas.core.advisor.AdviceListenerAdapter.isConditionMet(AdviceListenerAdapter.java:119)
at com.taobao.arthas.core.command.monitor200.TimeTunnelAdviceListener.afterFinishing(TimeTunnelAdviceListener.java:75)
at com.taobao.arthas.core.command.monitor200.TimeTunnelAdviceListener.afterReturning(TimeTunnelAdviceListener.java:58)
at com.taobao.arthas.core.advisor.AdviceListenerAdapter.afterReturning(AdviceListenerAdapter.java:57)
at com.taobao.arthas.core.advisor.SpyImpl.atExit(SpyImpl.java:68)
at java.arthas.SpyAPI.atExit(SpyAPI.java:64)
```
这个一般是什么原因导致的。