error: VM Exception while processing transaction: revert SafeMath: subtraction overflow on migrating crowdsale conract

Question

I am developing the token sale Dapp using openzepplin-solidity version 2.3.

While migrating, I got the following error.

Returned error: VM Exception while processing transaction: revert SafeMath: subtraction overflow -- Reason given: SafeMath: subtraction overflow

I am using

Truffle v5.0.22 (core: 5.0.22)
Solidity - 0.5.0 (solc-js)
Node v10.16.0
Web3.js v1.0.0-beta.37

Here is the link of my Dapp on Github.
I have 2 crowdsale contracts and 1 token contract.

SimpleToken.sol

pragma solidity 0.5.0;

import "../client/node_modules/openzeppelin-solidity/contracts/token/ERC20/ERC20Detailed.sol";
import "../client/node_modules/openzeppelin-solidity/contracts/token/ERC20/ERC20.sol";

contract SimpleToken is ERC20Detailed, ERC20 {
    uint256 public totalSupply_;
    mapping(address => uint256) balances;

   constructor(string memory _name, string memory _symbol, uint8 _decimals,   uint256 _amount)
   ERC20Detailed(_name, _symbol, _decimals)
   public {
    require(_amount > 0, "Amount has to be greater then 0");
    totalSupply_ = _amount * (10 ** 18);
    balances[msg.sender] = totalSupply_;
    emit Transfer(address(0), msg.sender, totalSupply_);
    }

}

And following is

PresaleCrowdsale.sol

pragma solidity 0.5.0;

import "../client/node_modules/openzeppelin-solidity/contracts/token/ERC20/ERC20.sol";
import "../client/node_modules/openzeppelin-solidity/contracts/crowdsale/Crowdsale.sol";
import "../client/node_modules/openzeppelin-solidity/contracts/crowdsale/validation/WhitelistCrowdsale.sol";
import "../client/node_modules/openzeppelin-solidity/contracts/crowdsale/emission/AllowanceCrowdsale.sol";

contract PresaleCrowdsale is WhitelistCrowdsale, AllowanceCrowdsale{
    constructor(uint256 _rate, address payable _wallet, ERC20 _token, address _tokenWallet)
    Crowdsale(_rate, _wallet, _token)
    AllowanceCrowdsale(_tokenWallet)
    public {  }
}

I can successfully migrating the token contract but while migrating the crowdsale contract I got the error in safeMath subtraction overflow

Following is the migration file

const SimpleToken = artifacts.require("SimpleToken.sol");
const PresaleCrowdsale = artifacts.require("PresaleCrowdsale.sol");

  module.exports = (deployer, network, [owner]) => deployer
 .then(() => deployer.deploy(PresaleCrowdsale, 10000, owner, SimpleToken.address, owner))
 .then(() => SimpleToken.deployed())
 .then(token => token.transfer(PresaleCrowdsale.address, '100000000000000000'));

EDIT
Add 2nd Migration file and transfer is not working. got the same error

revert SafeMath: subtraction overflow -- Reason given: SafeMath: subtraction overflow

const SimpleToken = artifacts.require("SimpleToken");
const GenericCrowdsale = artifacts.require("GenericCrowdsale");

module.exports =  async (deployer, network, [owner]) => {
await deployer.deploy(SimpleToken, "Tooploox", "TPX", 18, 21000000);

const now = Math.floor(Date.now() / 1000);

const day = 24 * 60 * 60;

const openingtime = now;

const closingtime = openingtime + 2 * day;

const rate = 1000;

await deployer.deploy(GenericCrowdsale, openingtime, closingtime, rate, owner, SimpleToken.address);

console.log("Transfering Token...");

const token = await SimpleToken.deployed();

await token.transfer(GenericCrowdsale.address, "20000000000000000000000000", {from: owner});
};

Please help me

Answer 1

You have an issue in your ERC20 contract implementation.

__balances, _totalSupply are private variables in the inherited ERC20 contract

The appropriate implementation should be

contract SimpleToken is ERC20Detailed, ERC20 {
    constructor(string memory _name, string memory _symbol, uint8 _decimals, uint256 _amount)
    ERC20Detailed(_name, _symbol, _decimals)
    public {
        require(_amount > 0, "Amount has to be greater then 0");
        uint256 initialSupply = _amount.mul(10 ** uint256(_decimals));
        _mint(msg.sender, initialSupply);
    }
}