C++11: array subscription fails to compile with "cannot assign `unique_ptr<foo_t>` to `foo_t *`"
In this function, no suitable conversion function, could you give me a hint? Thank you very much!

```cpp
blender_blend_t::dna_struct_t* blender_blend_t::file_block_t::sdna_struct() {
    if (f_sdna_struct)
        return m_sdna_struct;
    n_sdna_struct = true;
    if (sdna_index() != 0) {
        n_sdna_struct = false;
        m_sdna_struct = _root()->sdna_structs()->at(sdna_index()); //C2440 occurs
    }
    f_sdna_struct = true;
    return m_sdna_struct;
}
```