Remove closing external issues by reference error

app/models/merge_request.rb

@@ -605,6 +605,8 @@ class MergeRequest < ActiveRecord::Base
       self.merge_requests_closing_issues.delete_all
 
       closes_issues(current_user).each do |issue|
+        next if issue.is_a?(ExternalIssue)
+
         self.merge_requests_closing_issues.create!(issue: issue)
       end
     end

changelogs/unreleased/issue_36820.yml

@@ -0,0 +1,5 @@
+---
+title: Remove closing external issues by reference error
+merge_request:
+author:
+type: fixed

spec/models/merge_request_spec.rb

@@ -159,6 +159,7 @@ describe MergeRequest do
       before do
         subject.project.has_external_issue_tracker = true
         subject.project.save!
+        create(:jira_service, project: subject.project)
       end
 
       it 'does not cache issues from external trackers' do
@@ -166,6 +167,7 @@ describe MergeRequest do
         commit = double('commit1', safe_message: "Fixes #{issue.to_reference}")
         allow(subject).to receive(:commits).and_return([commit])
 
+        expect { subject.cache_merge_request_closes_issues!(subject.author) }.not_to raise_error
         expect { subject.cache_merge_request_closes_issues!(subject.author) }.not_to change(subject.merge_requests_closing_issues, :count)
       end