initializes the branches dropdown upon invalid entry

Added Changelog
added feature spec

app/assets/javascripts/dispatcher.js

@@ -383,6 +383,7 @@ import ProjectVariables from './project_variables';
           projectImport();
           break;
         case 'projects:pipelines:new':
+        case 'projects:pipelines:create':
           new NewBranchForm($('.js-new-pipeline-form'));
           break;
         case 'projects:pipelines:builds':

changelogs/unreleased/15588-fix-empty-dropdown-on-create-new-pipeline-in-case-of-validation-errors.yml

+---
+title: Initializes the branches dropdown when the 'Start new pipeline' failed due to validation errors
+merge_request: 15588
+author: Christiaan Van den Poel
+type: fixed

spec/features/projects/pipelines/pipelines_spec.rb

@@ -500,6 +500,18 @@ describe 'Pipelines', :js do
           end
           it { expect(page).to have_content('Missing .gitlab-ci.yml file') }
+          it 'creates a pipeline after first request failed and a valid gitlab-ci.yml file is available when trying again' do
+            click_button project.default_branch
+
+            stub_ci_pipeline_to_return_yaml_file
+
+            page.within '.dropdown-menu' do
+              click_link 'master'
+            end
+
+            expect { click_on 'Create pipeline' }
+              .to change { Ci::Pipeline.count }.by(1)
+          end
         end
       end
     end