How to withdraw all tokens from the my contract in solidity

I created a basic contract.But don't know the withdrawal function.I tried creating a basic function but it doesn't work.I want to write a function to withdraw eth from a contract .
can everyone help me write eth withdrawal function ?

This is my contract :

I have compiled successfully but i got 1 new error This is :Gas estimation failed you can help me ?

:1234: Code to reproduce

    function withdrawToken(address _tokenContract, uint256 _amount) external {
        IERC20 tokenContract = IERC20(_tokenContract);
        // transfer the token from address of this contract
        // to address of the user (executing the withdrawToken() function)
        tokenContract.transfer(msg.sender, _amount);

:computer: Environment

I used pragma solidity ^0.5.0;

Did your contract have _amount balance of that token when you tried to withdraw?