Occurs in Fiber with multiple instances of the React package (which can't share current owner).
const existing = useFiber(child, priority);
          existing.pendingProps = portal.children || [];
          existing.return = returnFiber;
          return existing;
        } else {
          deleteRemainingChildren(returnFiber, child);
          break;
        }
      } else {
        deleteChild(returnFiber, child);
      }
      child = child.sibling;
    }
    const created = createFiberFromPortal(
      portal,
      returnFiber.internalContextTag,
      priority,
    );
    created.return = returnFiber;
    return created;
  }
  // 
  // 
  // 
  function reconcileChildFibers(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChild: any,
    priority: PriorityLevel,
  ): Fiber | null {