Handle customization for all provisions

Don't throw an error if `all_dialogs[:customize][:fields]` doesn't exist

```
FATAL -- production: Error caught: [NoMethodError] undefined method `[]=' for nil:NilClass
       manageiq-ui-classic/app/controllers/application_controller/miq_request_methods.rb:42:in `prov_field_changed'
```

Author: kbrock

app/controllers/application_controller/miq_request_methods.rb

@@ -38,7 +38,7 @@ def prov_field_changed
 
       all_dialogs = @edit[:wf].get_all_dialogs rescue []
 
-      if @edit[:new] && @edit[:new][:addr_mode] && @edit[:new][:addr_mode][0] == "dhcp" && all_dialogs[:customize]
+      if @edit[:new] && @edit[:new][:addr_mode] && @edit[:new][:addr_mode][0] == "dhcp" && all_dialogs[:customize] && all_dialogs[:customize][:fields]
         all_dialogs[:customize][:fields][:hostname]&.[]=(:read_only, true)
         all_dialogs[:customize][:fields][:linux_host_name]&.[]=(:read_only, true)
         all_dialogs[:customize][:fields][:subnet_mask][:read_only] = true