Occurs in Fiber with multiple instances of the React package (which can't share current owner).
case REACT_PORTAL_TYPE:
            return placeSingleChild(
              reconcileSinglePortal(
                returnFiber,
                currentFirstChild,
                newChild,
                priority,
              ),
            );
        }
      }
    }
    if (disableNewFiberFeatures) {
      // 
      // 
      switch (returnFiber.tag) {
        case ClassComponent: {
          if (__DEV__) {
            const instance = returnFiber.stateNode;
            if (
              instance.render._isMockFunction &&
              typeof newChild === 'undefined'
            ) {
              // 
              // 
              break;
            }
          }
          const Component = returnFiber.type;
          invariant(