Restore job thrown exception when FE restarted

[EmmyMiao87]

Describe the bug
Restore job thrown following exception when FE restarted

2021-05-20 10:02:59,083 ERROR (backupHandler|24) [Daemon.run():118] daemon thread got exception. name: backupHandler
java.lang.IllegalStateException: null
        at com.google.common.base.Preconditions.checkState(Preconditions.java:492) ~[spark-dpp-1.0.0.jar:1.0.0]
        at org.apache.doris.backup.RestoreJob.genFileMapping(RestoreJob.java:1025) ~[palo-fe.jar:3.4.0]
        at org.apache.doris.backup.RestoreJob.checkAndPrepareMeta(RestoreJob.java:711) ~[palo-fe.jar:3.4.0]
        at org.apache.doris.backup.RestoreJob.run(RestoreJob.java:328) ~[palo-fe.jar:3.4.0]
        at org.apache.doris.backup.BackupHandler.runAfterCatalogReady(BackupHandler.java:170) ~[palo-fe.jar:3.4.0]
        at org.apache.doris.common.util.MasterDaemon.runOneCycle(MasterDaemon.java:58) ~[palo-fe.jar:3.4.0]
        at org.apache.doris.common.util.Daemon.run(Daemon.java:116) [palo-fe.jar:3.4.0]

To Reproduce
Steps to reproduce the behavior:

1. create a restore job
2. stop fe before restore job started
3. start fe
4. the restore job thrown exception

Desktop (please complete the following information):

• Version 0.14.7