Got this debugging another error. Filing here for reference. Original code: https://phabricator.intern.facebook.com/P59317705
if (returnFiber.lastEffect !== null) {
              returnFiber.lastEffect.nextEffect = workInProgress;
            } else {
              returnFiber.firstEffect = workInProgress;
            }
            returnFiber.lastEffect = workInProgress;
          }
        }
        if (__DEV__ && ReactFiberInstrumentation.debugTool) {
          ReactFiberInstrumentation.debugTool.onCompleteWork(workInProgress);
        }
        if (siblingFiber !== null) {
          
          return siblingFiber;
        } else if (returnFiber !== null) {
          
          workInProgress = returnFiber;
          continue;
        } else {
          
          isRootReadyForCommit = true;
          return null;
        }
      } else {
        
        
        
        const next = unwindWork(workInProgress);
        
        if (workInProgress.effectTag & DidCapture) {