[Bug] FE checkpoint thread start failed with NPE

[morningman]

Describe the bug
When FE start, error log in fe.log:

2020-06-07 11:10:35,705 ERROR 91 [Daemon.run():118] daemon thread got exception:
java.lang.NullPointerException: null
        at org.apache.doris.common.util.MasterDaemon.runOneCycle(MasterDaemon.java:45) ~[palo-fe.jar:?]
        at org.apache.doris.common.util.Daemon.run(Daemon.java:116) [palo-fe.jar:?]

To Reproduce
Not easy to reproduce

Solution

This bug is introduced by PR: #3784
Because the CHECKPOINT instance is not initialized before visiting it.

I will make a PR to fix this.