ExpandableListView在扩展组位置比先前扩展的组高一个的组时抛出NullPointerException

Question

expandableListView.setOnGroupExpandListener(new ExpandableListView.OnGroupExpandListener() {
        @Override
        public void onGroupExpand(int groupPosition) {
            if (prev != -1 && prev != groupPosition) {
                expandableListView.collapseGroup(prev);
                ImageView prevTriangle = (ImageView) expandableListView.getChildAt(prev).findViewById(R.id.triangleTop);
                prevTriangle.setVisibility(View.INVISIBLE);
            }
            ImageView triangle = (ImageView) expandableListView.getChildAt(groupPosition).findViewById(R.id.triangleTop);

            //NullPointerException Thrown here when clicking on one lower:
            triangle.setVisibility(View.VISIBLE);

            prev = groupPosition;
        }
    });
    expandableListView.setOnGroupCollapseListener(new ExpandableListView.OnGroupCollapseListener() {
        @Override
        public void onGroupCollapse(int groupPosition) {
            ImageView triangle = (ImageView) expandableListView.getChildAt(groupPosition).findViewById(R.id.triangleTop);
            triangle.setVisibility(View.INVISIBLE);
        }
    });

这是我的代码。我试图在父视图中定位ImageView并在可见/不可见之间切换。除了扩展比在其上方扩展的父级低一个位置的父级之外，它的效果很好。此时，ImageView引用为null，但仅限于此情况。

**当我说父母较低时，我的意思是它的小组位置高一个，或者上一个+ 1。