Fix error on changes tab when merge request cannot be created

app/controllers/projects/merge_requests/creations_controller.rb

@@ -43,11 +43,7 @@ class Projects::MergeRequests::CreationsController < Projects::MergeRequests::Ap
   end
 
   def diffs
-    @diffs = if @merge_request.can_be_created
-               @merge_request.diffs(diff_options)
-             else
-               []
-             end
+    @diffs = @merge_request.diffs(diff_options) if @merge_request.can_be_created
 
     @diff_notes_disabled = true
 

app/views/projects/merge_requests/creations/_diffs.html.haml

-= render "projects/diffs/diffs", diffs: @diffs, environment: @environment, show_whitespace_toggle: false
+- if @merge_request.can_be_created
+  = render "projects/diffs/diffs", diffs: @diffs, environment: @environment, show_whitespace_toggle: false
+- else
+  .nothing-here-block
+    This merge request cannot be created.

changelogs/unreleased/issue_41460.yml

+---
+title: Fix error on changes tab when merge request cannot be created
+merge_request:
+author:
+type: fixed

spec/controllers/projects/merge_requests/creations_controller_spec.rb

@@ -4,6 +4,16 @@ describe Projects::MergeRequests::CreationsController do
   let(:project) { create(:project, :repository) }
   let(:user)    { project.owner }
   let(:fork_project) { create(:forked_project_with_submodules) }
+  let(:get_diff_params) do
+    {
+      namespace_id: fork_project.namespace.to_param,
+      project_id: fork_project,
+      merge_request: {
+        source_branch: 'remove-submodule',
+        target_branch: 'master'
+      }
+    }
+  end
 
   before do
     fork_project.add_master(user)
@@ -13,18 +23,23 @@ describe Projects::MergeRequests::CreationsController do
 
   describe 'GET new' do
     context 'merge request that removes a submodule' do
-      render_views
-
       it 'renders new merge request widget template' do
-        get :new,
-            namespace_id: fork_project.namespace.to_param,
-            project_id: fork_project,
-            merge_request: {
-              source_branch: 'remove-submodule',
-              target_branch: 'master'
-            }
+        get :new, get_diff_params
+
+        expect(response).to be_success
+      end
+    end
+  end
+
+  describe 'GET diffs' do
+    context 'when merge request cannot be created' do
+      it 'does not assign diffs var' do
+        allow_any_instance_of(MergeRequest).to receive(:can_be_created).and_return(false)
+
+        get :diffs, get_diff_params.merge(format: 'json')
 
         expect(response).to be_success
+        expect(assigns[:diffs]).to be_nil
       end
     end
   end
@@ -37,14 +52,7 @@ describe Projects::MergeRequests::CreationsController do
     end
 
     it 'renders JSON including serialized pipelines' do
-      get :pipelines,
-          namespace_id: fork_project.namespace.to_param,
-          project_id: fork_project,
-          merge_request: {
-            source_branch: 'remove-submodule',
-            target_branch: 'master'
-          },
-          format: :json
+      get :pipelines, get_diff_params.merge(format: 'json')
 
       expect(response).to be_ok
       expect(json_response).to have_key 'pipelines'