Destroyable.sol:
import "./Ownable.sol";
pragma solidity 0.5.12;
contract Destroyable is Ownable{
function close() public onlyOwner {
selfdestruct(owner);
}
}
Changed owner variable to payable in Ownable.sol:
address payable public owner;
imported Destroyable and made HelloWorld inherit from it.
import "./Destroyable.sol";
pragma solidity 0.5.12;
contract HelloWorld is Ownable, Destroyable{
...
}