Click on a Build button and the error appears, for me on Kickstart default iPXE template - all the others are valid for some reason. Full stack:

Rendering Kickstart default iPXE
Review template error
 | Foreman::Exception: ERF42-1709 [Foreman::Exception]: Must supply an entity to find a medium provider
 | /home/lzap/work/foreman/app/registries/foreman/plugin/medium_providers_registry.rb:23:in `find_provider'
 | /home/lzap/work/foreman/app/models/concerns/host_template_helpers.rb:38:in `medium_provider'
 | /home/lzap/work/foreman/app/models/concerns/host_common.rb:83:in `url_for_boot'
 | /home/lzap/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/safemode-1.3.5/lib/safemode/jail.rb:31:in `method_missing'
 | Kickstart default iPXE:69:in `bind'
 | /home/lzap/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/safemode-1.3.5/lib/safemode.rb:51:in `eval'
 | /home/lzap/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/safemode-1.3.5/lib/safemode.rb:51:in `eval'
 | /home/lzap/work/foreman/lib/foreman/renderer/safe_mode_renderer.rb:7:in `render'
 | /home/lzap/work/foreman/lib/foreman/renderer/base_renderer.rb:16:in `render'
 | /home/lzap/work/foreman/lib/foreman/renderer.rb:45:in `render'
 | /home/lzap/work/foreman/app/models/template.rb:153:in `render'
 | /home/lzap/work/foreman/app/models/host/base.rb:351:in `render_template'
 | /home/lzap/work/foreman/app/services/host_build_status.rb:36:in `block in templates_status'
 | /home/lzap/work/foreman/app/services/host_build_status.rb:33:in `each'
 | /home/lzap/work/foreman/app/services/host_build_status.rb:33:in `templates_status'
 | /home/lzap/work/foreman/app/services/host_build_status.rb:15:in `check_all_statuses'
 | /home/lzap/work/foreman/app/models/host/managed.rb:733:in `build_status_checker'
 | /home/lzap/work/foreman/app/controllers/hosts_controller.rb:226:in `review_before_build'