I am having the same problem, I believe, when I try to add a NIC to a VM, it fails and returns some strange errors:
I follow the advice of upgrading to the latest host-client:

[root@localhost:/tmp] esxcli software vib install -v /tmp/esxui-signed-7119706.vib
Installation Result
   Message: Operation finished successfully.
   Reboot Required: false
   VIBs Installed: VMware_bootbank_esx-ui_1.24.0-7119706
   VIBs Removed: VMware_bootbank_esx-ui_1.21.0-5724747
   VIBs Skipped:
[root@localhost:/tmp]

After installing latest version 1.24 I cleared the chache and restarted the browser and tried to add the network adapter again and it failed, I also tried by adding a new port group but nothing seems to work:

Unhandled exception (1)

Cause: TypeError: $scope.NetworkData[0] is undefined
Version: 1.24.0
Build: 7119706
ESXi: 6.5.0
Browser: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Firefox/52.0

Exception stack:

$scope.addNetwork@https://192.168.255.64/ui/scripts/main.js:372:4852
virtualHardwareMenu<.onClick@https://192.168.255.64/ui/scripts/main.js:369:9246
directiveDefinitionObject.link/scope.clickAction@https://192.168.255.64/ui/scripts/main.js:460:9882
kb.prototype.functionCall/<@https://192.168.255.64/ui/scripts/main.js:321:23158
@https://192.168.255.64/ui/scripts/main.js:429:289
Pe/this.$get</n.prototype.$eval@https://192.168.255.64/ui/scripts/main.js:320:17493
Pe/this.$get</n.prototype.$apply@https://192.168.255.64/ui/scripts/main.js:320:17718
@https://192.168.255.64/ui/scripts/main.js:429:269
n.event.dispatch@https://192.168.255.64/ui/scripts/main.js:317:14412
n.event.add/r.handle@https://192.168.255.64/ui/scripts/main.js:317:11234


Regards,

Kenneth,