compact items that are consecutive

Example;

compactar({2,2,2,2})>>>> {2}

compactar({2,2,null,null,2,3})>>>>{2,null,2,3}

compactar({2,2,1,2})>>>>{2,1,2}

compactar({})>>>>{}

compactar({1,2,3,null,null})>>>>{1,2,3,null}

compactar({null})>>>>{null}

compactar(null)>>>>IllegalArgumentException

"The method returns [] instead of [1] when invoked with [1]."

I only get this error but I still do not know why, I made several changes but I can not find the result. The method basically seeks to compact the consecutive elements equal to a single appearance.

       public <E> PositionList<E> compactar (PositionList<E> lista) {
    PositionList<E> list= new NodePositionList<E>();
    Position<E> cursor=list.first();


    if(lista==null ) {
        throw new IllegalArgumentException();
    }// de if

    Position <E> cursor1= list.last();
    while(cursor!=null) {
            if(cursor.equals(lista.next(cursor))) {
                Position<E> borrar=cursor;
                cursor=lista.next(cursor);
                lista.remove(borrar);
                list.addLast(cursor.element());
            }// de if 2


    }// de bucle for 
    return list;
}// de compactar