Timeout when I try to read the descriptor

I tried to create a descriptor for a characteristic. I modified the app_blinky and added a ble_add_descr_params_t variable. After The Initialisation and definition of the ble_add_descr_params_t the descriptor is added with the descriptor_add funktion. Then the descriptor is added to the add_char_params.p_user_descr. The characteristic_add function is called after this.

When I run the code I don't ge an error report or anything else. The NRF_LOG just says :

<info> app: Blinky example started

<info> app: Connected

When I connet with an other nRF Board via the nRF Connect Bluetooth Low Energy App, I can see that the characteristic has something "attatched". But if I want to read the descriptor of the characteristic I get a timeout. It's the same of a Phone.

I'm not sure if I called the function right.

	err_code =descriptor_add(p_lbs->service_handle, & char_params, &p_lbs->service_handle);