问无法从另一(ERC20)合同调用ERC20 20‘S ERC20()

EN

pragma solidity ^0.4.24;

contract ERC20Basic{

function balanceOf(address _owner) external constant returns (uint256 balance);
function transfer(address to, uint256 value) public returns (bool);


}

library SafeMath {//SafeMath Code}

contract TokenTransferContract{

using SafeMath for uint256;

address _owner;
address _tokenAddress;
ERC20Basic erc20Instance;

constructor () public {

    _owner = msg.sender; //Contract distributor is the owner of this contract.

}

// Setting the instance to call functions from different contract (In this case ERC20 Token's function)

function setTokenInstance(address _tokenContractAddress) public {

    _tokenAddress = _tokenContractAddress;
    erc20Instance = ERC20Basic(_tokenContractAddress);

}

//transfer function works.

function transferContractTokenToOwner(uint256 _tokenAmount) public {
    require(_owner == msg.sender);

    erc20Instance.transfer(_owner, _tokenAmount); //Transfering the amount of token that was allocated with the address of this(UserTokenWalletContract) address to Owner(EOA).

}

function getAmountOfTokenOfOwner() view public returns (uint256 _amount){
    require(_owner == msg.sender);

    _amount = (erc20Instance.balanceOf(_owner)).div(1000000000000000000);

}

function getAmountOfTokenOfContract() view public returns (uint256 _amount){

    require(_owner == msg.sender);

    _amount = (erc20Instance.balanceOf(address(this))).div(1000000000000000000);

}

 function noneDecimalGetAmountOfTokenOfOwner() view public returns (uint256 _amount){
    require(_owner == msg.sender);

    _amount = erc20Instance.balanceOf(_owner);

}

function noneDecimalgetAmountOfTokenOfWalletContract() view public returns (uint256 _amount){
    require(_owner == msg.sender);

    _amount = erc20Instance.balanceOf(address(this));

}


function getOwnerAddress() view public returns (address _ownerAddress){
    _ownerAddress = _owner;

}

function getContractAddress() view public returns (address _contractAddress){
    _contractAddress = address(this);

}

function getContractInstanceAddress() view public returns (address _contractInstanceAddress){ 
    _contractInstanceAddress = _tokenAddress;

}

}