Installing Knative without domain name gives meaningless error

Summary

When I try to install Knative by clicking the Install button, I get a useless error message

Steps to reproduce

1. Add a cluster to GitLab.
2. Hit the Install button beside Helm
3. Hit the Install button beside Knative

Example Project

https://gitlab.com/groups/knative-examples/-/clusters/85746

What is the current bug behavior?

What is the expected correct behavior?

Both of the following:

1. A meaningful error message, like Please, provide a domain name
2. If the cluster base domain is set, then pre-fill the knative domain name with its value, and no error appears at all

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code as it's tough to read otherwise.)

Output of checks

(If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com)

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:env:info)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)