```

    def process_question(self, query):
        selected_files = self.get_selected_files(self.file_tree)
        node_ids = [n.node_id for n in self.index.docstore.docs.values() if n.metadata['file_name'] in selected_files]
        retriever = VectorIndexRetriever(self.index, node_ids=node_ids, similarity_top_k=3)
        custom_template = (...)
        query_engine = RetrieverQueryEngine.from_args(
            retriever=retriever,
            response_mode="compact",
            prompt_template=PromptTemplate(custom_template)
        )
        chat_engine = self.index.as_chat_engine()
        response = chat_engine.chat(query)