Contract API could not return multiple parameters

I created smartcontract and has been generated using FireFly API Interface.
This is my smartcontract code:

```
contract Fromzero {
struct Track {
address createdBy;
string materialNumber;
}
Track track;
function gettrack() public view returns (address createdBy, string memory materialNumber) {
return (track.createdBy, track.materialNumber);
}
}
```
expected output:
```
{
"createdBy": "string",
"materialNumber": "string"
}
```
but got:
```
{
"output": null
}
```
I have tried using remix and its working.

Could you all help me please?