Do not raise if selected item is not found

As noticed in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/80681#note_862019211
and
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/81370#note_854661919,
the result of raising if the selected item cannot be found is a 500
error. Considering this can be triggered by user behaviour (i.e.,
changing the URL) the only sensible way to handle this is to select the
first item, as was already the case for `selected: nil`.

Addresses https://gitlab.com/gitlab-org/gitlab/-/issues/354424.

app/helpers/listbox_helper.rb

@@ -16,8 +16,10 @@ module ListboxHelper
   # the sort key), `text` is the user-facing string for the item, and `href` is
   # the path to redirect to when that item is selected.
   #
-  # The `selected` parameter is the currently selected `value`, and must
-  # correspond to one of the `items`, or be `nil`. When `selected.nil?`, the first item is selected.
+  # The `selected` parameter is the currently selected `value`, and should
+  # correspond to one of the `items`, or be `nil`. When `selected.nil?` or
+  # a value which does not correspond to one of the items, the first item is
+  # selected.
   #
   # The final parameter `html_options` applies arbitrary attributes to the
   # returned tag. Some of these are passed to the underlying Vue component as
@@ -37,9 +39,12 @@ module ListboxHelper
       webpack_bundle_tag 'redirect_listbox'
     end
 
-    selected ||= items.first[:value]
     selected_option = items.find { |opt| opt[:value] == selected }
-    raise ArgumentError, "cannot find #{selected} in #{items}" unless selected_option
+
+    unless selected_option
+      selected_option = items.first
+      selected = selected_option[:value]
+    end
 
     button = button_tag(type: :button, class: DROPDOWN_BUTTON_CLASSES) do
       content_tag(:span, selected_option[:text], class: DROPDOWN_INNER_CLASS) +

spec/helpers/listbox_helper_spec.rb

@@ -65,10 +65,13 @@ RSpec.describe ListboxHelper do
     end
 
     context 'when selected does not match any item' do
-      let(:selected) { 'qux' }
+      where(selected: [nil, 'qux'])
 
-      it 'raises an error' do
-        expect { subject }.to raise_error(ArgumentError, /cannot find qux/)
+      with_them do
+        it 'selects first item' do
+          expect(subject.at_css('button').content).to eq('Foo')
+          expect(subject.attributes['data-selected'].value).to eq('foo')
+        end
       end
     end
   end