Resolve "catch entering username as an email instead of returning a 500 error"

ee/app/views/admin/users/_limits.html.haml

+- return unless @user.persisted?
+
 = f.fields_for :namespace do |namespace_form|
   = namespace_form.hidden_field :id
   %fieldset

ee/changelogs/unreleased/7044-fix-admin-user-create-error.yml

+---
+title: Fixes 500 error on user creation from admin panel with spaced username
+merge_request: 6804
+author: Jacopo Beschi @jacopo-beschi
+type: fixed

spec/features/admin/admin_users_spec.rb

@@ -68,10 +68,12 @@ describe "Admin::Users" do
   end
 
   describe "GET /admin/users/new" do
+    let(:user_username) { 'bang' }
+
     before do
       visit new_admin_user_path
       fill_in "user_name", with: "Big Bang"
-      fill_in "user_username", with: "bang"
+      fill_in "user_username", with: user_username
       fill_in "user_email", with: "bigbang@mail.com"
     end
 
@@ -112,6 +114,17 @@ describe "Admin::Users" do
       expect(email.text_part.body).to have_content(user.email)
       expect(email.text_part.body).to have_content('password')
     end
+
+    context 'username contains spaces' do
+      let(:user_username) { 'Bing bang' }
+
+      it "doesn't create the user and shows an error message" do
+        expect { click_button "Create user" }.to change {User.count}.by(0)
+
+        expect(page).to have_content('The form contains the following error')
+        expect(page).to have_content('Username can contain only letters, digits')
+      end
+    end
   end
 
   describe "GET /admin/users/:id" do