Fail to upgrade NetScaler  to NS 13.1 61.25 or NS 14.1 60.52 with error: "'null' expected, at character offset 0 (before "nsinternal login dis...") at ./installns_legacy line 4186 or ./installns line 4050"

When executing the following command, we can see a specific error message. From the logs, we can tell that the connection to 127.0.0.1 fails because the file /nsconfig/ssh/ns_comm_key is missing.

# /netscaler/nsremotexec 127.0.0.1 -exec "clicmdnitro -clicmd 'show ssl service' -execute"
   nsinternal login disabled, connecting to 127.0.0.1...
   Error in accessing the local identity file (/nsconfig/ssh/ns_comm_key -- No such file or directory), while       connecting to 127.0.0.1, for remotely running showtechsupport

Solution 1: Enable internaluserlogin
                  set ns param -internaluserlogin ENABLED
Solution 2: Follow link to regenerate the key ns_comm_key
                  https://docs.netscaler.com/en-us/netscaler-hardware-platforms/mpx/netscaler-initial-configuration.html#generate-the-keys-and-connect-to-a-remote-netscaler-appliance-by-using-ssh-keys

Fail to upgrade NetScaler  to NS 13.1 61.25 or NS 14.1 60.52 with error: "'null' expected, at character offset 0 (before "nsinternal login dis...") at ./installns_legacy line 4186 or ./installns line 4050"