Fix bug causing repo mirror settings UI to break

Fixes an exception relating to the new project
cleanup settings in 11.6 that causes the mirror
repo settings UI to become unusable.

app/assets/javascripts/lib/utils/file_upload.js

 export default (buttonSelector, fileSelector) => {
   const btn = document.querySelector(buttonSelector);
   const fileInput = document.querySelector(fileSelector);
+
+  if (!btn || !fileInput) return;
+
   const form = btn.closest('form');
 
   btn.addEventListener('click', () => {

changelogs/unreleased/fix-repo-settings-file-upload-error.yml

+---
+title: Fix bug causing repository mirror settings UI to break
+merge_request: 23712
+author:
+type: fixed

spec/javascripts/lib/utils/file_upload_spec.js

@@ -9,7 +9,10 @@ describe('File upload', () => {
         <span class="js-filename"></span>
       </form>
     `);
+  });
 
+  describe('when there is a matching button and input', () => {
+    beforeEach(() => {
       fileUpload('.js-button', '.js-input');
     });
 
@@ -33,4 +36,29 @@ describe('File upload', () => {
 
       expect(document.querySelector('.js-filename').textContent).toEqual('index.js');
     });
+  });
+
+  it('fails gracefully when there is no matching button', () => {
+    const input = document.querySelector('.js-input');
+    const btn = document.querySelector('.js-button');
+    fileUpload('.js-not-button', '.js-input');
+
+    spyOn(input, 'click');
+
+    btn.click();
+
+    expect(input.click).not.toHaveBeenCalled();
+  });
+
+  it('fails gracefully when there is no matching input', () => {
+    const input = document.querySelector('.js-input');
+    const btn = document.querySelector('.js-button');
+    fileUpload('.js-button', '.js-not-input');
+
+    spyOn(input, 'click');
+
+    btn.click();
+
+    expect(input.click).not.toHaveBeenCalled();
+  });
 });