Fix pull/push mirror authentication fields

Added a null-safe check around toggles on the page to prevent errors

Changelog: fixed

app/assets/javascripts/protected_branches/protected_branch_edit.js

@@ -31,36 +31,40 @@ export default class ProtectedBranchEdit {
     const wrap = this.$wrap.get(0);
 
     const forcePushToggle = initToggle(wrap.querySelector('.js-force-push-toggle'));
-    forcePushToggle.$on('change', (value) => {
-      forcePushToggle.isLoading = true;
-      forcePushToggle.disabled = true;
-      this.updateProtectedBranch(
-        {
-          allow_force_push: value,
-        },
-        () => {
-          forcePushToggle.isLoading = false;
-          forcePushToggle.disabled = false;
-        },
-      );
-    });
-
-    if (this.hasLicense) {
-      const codeOwnerToggle = initToggle(wrap.querySelector('.js-code-owner-toggle'));
-      codeOwnerToggle.$on('change', (value) => {
-        codeOwnerToggle.isLoading = true;
-        codeOwnerToggle.disabled = true;
+    if (forcePushToggle) {
+      forcePushToggle.$on('change', (value) => {
+        forcePushToggle.isLoading = true;
+        forcePushToggle.disabled = true;
         this.updateProtectedBranch(
           {
-            code_owner_approval_required: value,
+            allow_force_push: value,
           },
           () => {
-            codeOwnerToggle.isLoading = false;
-            codeOwnerToggle.disabled = false;
+            forcePushToggle.isLoading = false;
+            forcePushToggle.disabled = false;
           },
         );
       });
     }
+
+    if (this.hasLicense) {
+      const codeOwnerToggle = initToggle(wrap.querySelector('.js-code-owner-toggle'));
+      if (codeOwnerToggle) {
+        codeOwnerToggle.$on('change', (value) => {
+          codeOwnerToggle.isLoading = true;
+          codeOwnerToggle.disabled = true;
+          this.updateProtectedBranch(
+            {
+              code_owner_approval_required: value,
+            },
+            () => {
+              codeOwnerToggle.isLoading = false;
+              codeOwnerToggle.disabled = false;
+            },
+          );
+        });
+      }
+    }
   }
 
   updateProtectedBranch(formData, callback) {

spec/frontend/protected_branches/protected_branch_edit_spec.js

@@ -73,6 +73,21 @@ describe('ProtectedBranchEdit', () => {
     });
   });
 
+  describe('when toggles are not available in the DOM on page load', () => {
+    beforeEach(() => {
+      create({ hasLicense: true });
+      setFixtures('');
+    });
+
+    it('does not instantiate the force push toggle', () => {
+      expect(findForcePushToggle()).toBe(null);
+    });
+
+    it('does not instantiate the code owner toggle', () => {
+      expect(findCodeOwnerToggle()).toBe(null);
+    });
+  });
+
   describe.each`
     description     | checkedOption               | patchParam                        | finder
     ${'force push'} | ${'forcePushToggleChecked'} | ${'allow_force_push'}             | ${findForcePushToggle}