adds guard clause for readme model method and initializes a new view with proper stylesheets

app/helpers/preferences_helper.rb

@@ -50,7 +50,9 @@ module PreferencesHelper
   end
 
   def default_project_view
-    return 'readme' unless current_user
+    unless current_user
+      return @repository.head_commit ? 'readme' : 'activity'
+    end
 
     user_view = current_user.project_view
 

app/models/repository.rb

@@ -554,6 +554,8 @@ class Repository
   end
 
   def readme
+    return unless head_commit
+
     cache.fetch(:readme) { tree(:head).readme }
   end
 

changelogs/unreleased/23990-project-show-error-when-empty-repo.yml

+---
+title: 500 error on project show when user is not logged in and project is still empty
+merge_request: 7376
+author: 

spec/helpers/preferences_helper_spec.rb

@@ -85,4 +85,25 @@ describe PreferencesHelper do
         and_return(double('user', messages))
     end
   end
+
+  describe 'default_project_view' do
+    let(:project) { create(:project) }
+
+    context 'user not signed in' do
+      before do
+        stub_user
+        @repository = project.repository
+      end
+
+      it 'returns readme view if repository is not empty' do
+        expect(helper.default_project_view).to eq('readme')
+      end
+
+      it 'returns activity if repository is empty' do
+        expect(@repository).to receive(:head_commit).and_return(nil)
+
+        expect(helper.default_project_view).to eq('activity')
+      end
+    end
+  end
 end