the cmd exits with exception

[imotai]

Bug description

@db3.network🚀🚀🚀
WARNING, db3 will generate private key and save it to /home/jackwang/.db3/user.key
restore the key with addr 0x97b478ac2bef8762c2ecc08417f35838c4bf73fc
>account
 total bills  | storage used | mutation | querys | credits 
 0.000000 db3 | 0.00         | 0        | 0      | 10 db3 
>put k1 n1
no enough command, e.g. put n1 k1 v1 k2 v2 k3 v3
>put ns k1 v1
submit mutation to mempool done!
>get ns k1
start a session before query
>session start
no enough command, e.g. put n1 k1 v1 k2 v2 k3 v3
>help
the help of db3 command
help    show all command
put     write pairs of key and value to db3 e.g. put ns1 key1 value1 key2 values
del     delete key from db3                 e.g. del ns1 key1 key2
get     get value from db3                  e.g. get ns1 key1 key2
range   get a range from db3                e.g. range ns1 start_key end_key
account get balance of current account
blocks  get latest blocks
session info        get session info    e.g session info
session restart     restart session             e.g session restart


>session restart
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /media/jackwang/993f27f5-6c63-4a2f-ab72-06639127f882/opensource/db3/src/cmd/src/lib.rs:189:57
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

the bug was found in the main branch

[jingchen2222]

Fixed by #163 `session restart` was removed