Fix extracting Sentry external URL when URL is nil

app/models/error_tracking/project_error_tracking_setting.rb

@@ -128,7 +128,7 @@ module ErrorTracking
     # ->
     # http://HOST/ORG/PROJECT
     def self.extract_sentry_external_url(url)
-      url.sub('api/0/projects/', '')
+      url&.sub('api/0/projects/', '')
     end
 
     def api_host

changelogs/unreleased/pl-nil-guard-extract-sentry-external-url.yml

+---
+title: Fix extracting Sentry external URL when URL is nil
+merge_request: 23162
+author:
+type: fixed

spec/models/error_tracking/project_error_tracking_setting_spec.rb

@@ -64,6 +64,22 @@ describe ErrorTracking::ProjectErrorTrackingSetting do
     end
   end
 
+  describe '.extract_sentry_external_url' do
+    subject { described_class.extract_sentry_external_url(sentry_url) }
+
+    describe 'when passing a URL' do
+      let(:sentry_url) { 'https://sentrytest.gitlab.com/api/0/projects/sentry-org/sentry-project' }
+
+      it { is_expected.to eq('https://sentrytest.gitlab.com/sentry-org/sentry-project') }
+    end
+
+    describe 'when passing nil' do
+      let(:sentry_url) { nil }
+
+      it { is_expected.to be_nil }
+    end
+  end
+
   describe '#sentry_external_url' do
     let(:sentry_url) { 'https://sentrytest.gitlab.com/api/0/projects/sentry-org/sentry-project' }