Occurs in Fiber with multiple instances of the React package (which can't share current owner).
returnFiber: Fiber,
    currentFirstChild: Fiber,
  ): Map<string | number, Fiber> {
    
    
    
    const existingChildren: Map<string | number, Fiber> = new Map();
    let existingChild = currentFirstChild;
    while (existingChild !== null) {
      if (existingChild.key !== null) {
        existingChildren.set(existingChild.key, existingChild);
      } else {
        existingChildren.set(existingChild.index, existingChild);
      }
      existingChild = existingChild.sibling;
    }
    return existingChildren;
  }
  function useFiber(fiber: Fiber, priority: PriorityLevel): Fiber {