Occurs in Fiber with multiple instances of the React package (which can't share current owner).
returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    yieldNode: ReactYield,
    priority: PriorityLevel,
  ): Fiber {
    // 
    let child = currentFirstChild;
    if (child !== null) {
      if (child.tag === YieldComponent) {
        deleteRemainingChildren(returnFiber, child.sibling);
        const existing = useFiber(child, priority);
        existing.type = yieldNode.value;
        existing.return = returnFiber;
        return existing;
      } else {
        deleteRemainingChildren(returnFiber, child);
      }
    }
    const created = createFiberFromYield(
      yieldNode,
      returnFiber.internalContextTag,
      priority,
    );
    created.type = yieldNode.value;
    created.return = returnFiber;
    return created;
  }
  function reconcileSinglePortal(