Got this debugging another error. Filing here for reference. Original code: https://phabricator.intern.facebook.com/P59317705
return siblingFiber;
        } else if (returnFiber !== null) {
          
          workInProgress = returnFiber;
          continue;
        } else {
          return null;
        }
      }
    }
    
    
    
    return null;
  }
  function performUnitOfWork(workInProgress: Fiber): Fiber | null {
    
    
    
    
    const current = workInProgress.alternate;
    
    startWorkTimer(workInProgress);
    if (__DEV__) {
      ReactDebugCurrentFiber.setCurrentFiber(workInProgress);
    }
    if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {
      stashedWorkInProgressProperties = Object.assign({}, workInProgress);
    }
    let next = beginWork(current, workInProgress, nextRenderExpirationTime);