FIX: Ensure that user exists before migrating.

app/jobs/onceoff/migrate_from_slack_official.rb

@@ -105,7 +105,8 @@ module Jobs
       end
 
       if !(slack_discourse_username = site_settings_value('slack_discourse_username')).nil?
-        SiteSetting.chat_integration_discourse_username = slack_discourse_username
+        username = User.find_by(username: slack_discourse_username.downcase)&.username
+        SiteSetting.chat_integration_discourse_username = (username || Discourse.system_user.username)
       end
     end
 

spec/jobs/onceoff/migrate_from_slack_official_spec.rb

@@ -41,6 +41,19 @@ RSpec.describe Jobs::DiscourseChatMigrateFromSlackOfficial do
       expect(SiteSetting.chat_integration_slack_enabled).to eq(true)
       expect(SiteSetting.chat_integration_enabled).to eq(true)
     end
+
+    describe 'when slack_discourse_username is not valid' do
+      before do
+        SiteSetting.find_by(name: 'slack_discourse_username').update!(value: 'someguy')
+      end
+
+      it 'should default to the system user' do
+        described_class.new.execute_onceoff({})
+
+        expect(SiteSetting.chat_integration_discourse_username)
+          .to eq(Discourse.system_user.username)
+      end
+    end
   end
 
   describe 'when a uncategorized filter is present' do