Fix bug where pull request cannot be viewed if pullRequest.headRepository is null.

lib/controllers/issueish-detail-controller.js

@@ -153,6 +153,12 @@ export class BareIssueishDetailController extends React.Component {
       return this.checkoutOp.disable(checkoutStates.DISABLED, 'Checking out...');
     }
 
+    // determine if pullRequest.headRepository is null
+    // this can happen if a repository has been deleted.
+    if (!pullRequest.headRepository) {
+      return this.checkoutOp.disable(checkoutStates.DISABLED, 'Pull request head repository does not exist');
+    }
+
     // Determine if we already have this PR checked out.
 
     const headPush = this.props.branches.getHeadBranch().getPush();

test/controllers/issueish-detail-controller.test.js

@@ -109,6 +109,15 @@ describe('IssueishDetailController', function() {
       assert.isFalse(op1.isEnabled());
       assert.strictEqual(op1.getMessage(), 'Rebase in progress');
     });
+    it('is disabled if pullRequest.headRepository is null', function() {
+      const props = issueishDetailControllerProps({}, {});
+      props.repository.pullRequest.headRepository = null;
+      const wrapper = shallow(buildApp({}, {...props}));
+      const op = wrapper.find('Relay(BarePullRequestDetailView)').prop('checkoutOp');
+      assert.isFalse(op.isEnabled());
+      assert.strictEqual(op.getMessage(), 'Pull request head repository does not exist');
+    });
+
 
     it('is disabled if the current branch already corresponds to the pull request', function() {
       const upstream = Branch.createRemoteTracking('remotes/origin/feature', 'origin', 'refs/heads/feature');