message: Added defensive check
  Don't think it can ever happen but don;'t ever want to unmount all (or any) scenes with 0 reactPop time.  Never even seen poppedItem null but still better to check
diff: @@ -33,7 +33,7 @@ class PopSync<T> extends React.Component<PopSyncProps<T>, any> {
  popNative(key: string) {
  this.setState(({items: prevItems}) => {
  var poppedItem = prevItems.filter(item => item.key === key)[0];
  - if (!poppedItem)
  + if (!poppedItem || !poppedItem.reactPop)
  return null;
  var items = prevItems.filter(({reactPop, index}) => (
  reactPop !== poppedItem.reactPop || index > poppedItem.index
  