Flight Tasks tutorial does not work

Hi, I’m trying to give a desired position command a multicopter vehicle.
I think that Flight Tasks seems a tool that I need.
However, I encountered some errors when following the instruction.
My code can be found in this commit.

Note that I slightly changed the instruction from

...
// manual position control
...
switch (_param_mpc_pos_mode.get()) {
  ...
  case 3:
     error = switchTask(FlightTaskIndex::ManualPositionSmoothVel);
     break;
  case 5: // Add case for new task: MyTask
     error =  _flight_tasks.switchTask(FlightTaskIndex::MyTask);
     break;

to

...
// manual position control
...
switch (_param_mpc_pos_mode.get()) {
  ...
  case 3:
     error = switchTask(FlightTaskIndex::ManualPositionSmoothVel);
     break;
  case 5: // Add case for new task: MyTask
     error = switchTask(FlightTaskIndex::MyTask);
     break;

due to another error.

With this, cmake .. at ./build and make px4_sitl gazebo at ./ yields

...
// manual position control
...
switch (_param_mpc_pos_mode.get()) {
  ...
  case 3:
     error = switchTask(FlightTaskIndex::ManualPositionSmoothVel);
     break;
  case 5: // Add case for new task: MyTask
     error =  _flight_tasks.switchTask(FlightTaskIndex::MyTask);
     break;

Notes

environment