Getting a 0x3401=BLE_ERROR_GATTS_SYS_ATTR_MISSING from sd_ble_gatts_hvx

So I'm trying to write a value to a characteristic I have set up as in the code below (figure 1), using the code further below (figure 2).

The sd_ble_gatts_hvx call is coming back with BLE_ERROR_GATTS_SYS_ATTR_MISSING.  I've read here 

https://devzone.nordicsemi.com/f/nordic-q-a/1948/strange-error-code-13313-0x3401-returned-by-sd_ble_gatts_hvx

that this is perhaps normally handled with the BLE_GATTS_EVT_SYS_ATTR_MISSING event, and I'm supposed to make a call like I have in my figure 3 below.

However, this event is not being called.  I'm not sure if that's the cause of my error.

So, is there something wrong with my setup, my write, or have I somehow misrouted my event handling of BLE_GATTS_EVT_SYS_ATTR_MISSING?

(I have refactored quite a bit from the example, may have broken something.)

Thanks for any help!

Figure 1: setting up the characteristic

            //////////////////////////////////////////
            // Add State characteristic BLE_UUID_SMILE_STATE
            ////////////////////////////////////////// 

            memset(&add_char_params, 0, sizeof(add_char_params));
            add_char_params.uuid                      = BLE_UUID_SMILE_STATE;
            add_char_params.uuid_type                 = service_uuid.type;
            add_char_params.max_len                   = 10;
            add_char_params.init_len                  = sizeof(uint8_t);
            add_char_params.is_var_len                = true;
                        
            add_char_params.char_props.broadcast      = 0;
            add_char_params.char_props.read           = 1;
            add_char_params.char_props.write_wo_resp  = 0;
            add_char_params.char_props.write          = 0;
            add_char_params.char_props.notify         = 1;
            add_char_params.char_props.indicate       = 0;
            add_char_params.char_props.auth_signed_wr = 0;            
            add_char_params.read_access               = SEC_OPEN;
            add_char_params.write_access              = SEC_OPEN;
            add_char_params.cccd_write_access         = SEC_OPEN;

            err_code = characteristic_add(p_our_service->service_handle, &add_char_params,&(p_our_service->characteristichandles_State));            
            if (err_code != NRF_SUCCESS)             {                 RETURN_FAULT(10,err_code)   

Figure 2: code that writes

            uint16_t length = 10;
            uint8_t state_bytes[10];
            state_bytes[0] = 0xFF & (state >> 8);
            state_bytes[1] = 0xFF & (state >> 0);

            ble_gatts_hvx_params_t hvx_params;

            memset(&hvx_params, 0, sizeof(hvx_params));

            hvx_params.handle = m_our_service.characteristichandles_State.value_handle;
            hvx_params.type   =   BLE_GATT_HVX_NOTIFICATION;
            hvx_params.offset = 0;
            hvx_params.p_len  = &length;
            hvx_params.p_data = state_bytes;

            uint32_t err_code = sd_ble_gatts_hvx(  m_conn_handle, &hvx_params);

Figure 3: my handler for BLE_ERROR_GATTS_SYS_ATTR_MISSING

static void ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context)
{
    uint32_t err_code;

    switch (p_ble_evt->header.evt_id)
    {
// ... 

        case BLE_GATTS_EVT_SYS_ATTR_MISSING:
            
            UART_SendString("[BLE event: BLE_GATTS_EVT_SYS_ATTR_MISSING]");    
        
            // No system attributes have been stored.
            err_code = sd_ble_gatts_sys_attr_set(m_conn_handle, NULL, 0, 0);
            APP_ERROR_CHECK(err_code);
            break;

  

    }
}