Fix 500 error when editing conflicts with approvals enabled

When an MR or issue and there's a conflict, we rescue the StaleObjectError and
render the edit page again. However, for MRs in EE, we need to ensure that
@suggested_approvers is set before that happens, otherwise the view will fail to
render.

app/controllers/projects/merge_requests_controller.rb

@@ -311,6 +311,8 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       end
     end
   rescue ActiveRecord::StaleObjectError
+    set_suggested_approvers if request.format.html?
+
     render_conflict_response
   end
 

changelogs/unreleased-ee/2618-actionview-template-error-undefined-method-any-for-nil-nilclass.yml

+---
+title: Fix 500 error when approvals are enabled and editing an MR conflicts with another
+  edit
+merge_request:
+author:

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -456,6 +456,12 @@ describe Projects::MergeRequestsController do
       it_behaves_like 'update invalid issuable', MergeRequest
     end
 
+    context 'when the merge request requires approval' do
+      before { project.update_attributes(approvals_before_merge: 1) }
+
+      it_behaves_like 'update invalid issuable', MergeRequest
+    end
+
     context 'the approvals_before_merge param' do
       before { project.update_attributes(approvals_before_merge: 2) }
 

spec/support/update_invalid_issuable.rb

@@ -30,6 +30,10 @@ shared_examples 'update invalid issuable' do |klass|
 
       expect(response).to render_template(:edit)
       expect(assigns[:conflict]).to be_truthy
+
+      if klass == MergeRequest && issuable.requires_approve?
+        expect(assigns[:suggested_approvers]).to be_an(Array)
+      end
     end
 
     it 'renders json error message when format is json' do