Got this debugging another error. Filing here for reference. Original code: https://phabricator.intern.facebook.com/P59317705
root.finishedWork = null;
        finishedWork = renderRoot(root, expirationTime, false);
        if (finishedWork !== null) {
          
          completeRoot(root, finishedWork, expirationTime);
        }
      }
    } else {
      
      let finishedWork = root.finishedWork;
      if (finishedWork !== null) {
        
        completeRoot(root, finishedWork, expirationTime);
      } else {
        root.finishedWork = null;
        finishedWork = renderRoot(root, expirationTime, true);
        if (finishedWork !== null) {
          
          
          if (!shouldYield()) {
            
            completeRoot(root, finishedWork, expirationTime);
          } else {
            
            
            root.finishedWork = finishedWork;
          }
        }
      }
    }