Save and restore expanded/collapsed state of an ExpandableListActivity

I have an ExpandableListActivity (using a SimpleCursorTreeAdapter) which starts another activity when the user clicks on a child-element. When pressing the back-button in the new activity all the list-items are collapsed again. How do I save the expanded state of the ExpandableListActivity and restore it again.

I already tried to implemented onSaveInstanceState() and onRestoreInstanceState() like this...

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    Parcelable listState = getExpandableListView().onSaveInstanceState();
    outState.putParcelable("ListState", listState);
}


@Override
protected void onRestoreInstanceState(Bundle state) {
    super.onRestoreInstanceState(state);
    Parcelable listState = state.getParcelable("ListState");
    getExpandableListView().onRestoreInstanceState(listState);
}

...but onRestoreInstanceState() gets never called. I also tried to restore the state in the onCreate() method but it isn't called as well:

if (savedInstanceState != null) {
    Parcelable listState = savedInstanceState.getParcelable("ListState");
    getExpandableListView().onRestoreInstanceState(listState);
}