Crowdsale Gas limit problem

This complete code brings Gas limit error saying. What are code problems?

creation of Crowdsale pending... creation of Crowdsale errored: Gas required exceeds limit: 3000000. An important gas estimation might also be the sign of a problem in the contract code. Please check loops and be sure you did not sent value to a non payable function (that's also the reason of strong gas estimation).

``````pragma solidity ^0.4.17;

/**
* @title ERC20Basic
* @dev Simpler version of ERC20 interface
* @dev see https://github.com/ethereum/EIPs/issues/179
*/
contract ERC20Basic {
uint256 public totalSupply;
function balanceOf(address who) public view returns (uint256);
function transfer(address to, uint256 value) public returns (bool);
}

/**
* @title SafeMath
* @dev Math operations with safety checks that throw on error
*/
library SafeMath {
function mul(uint256 a, uint256 b) internal pure returns (uint256) {
if (a == 0) {
return 0;
}
uint256 c = a * b;
assert(c / a == b);
return c;
}

function div(uint256 a, uint256 b) internal pure returns (uint256) {
// assert(b > 0); // Solidity automatically throws when dividing by 0
uint256 c = a / b;
// assert(a == b * c + a % b); // There is no case in which this doesn't hold
return c;
}

function sub(uint256 a, uint256 b) internal pure returns (uint256) {
assert(b <= a);
return a - b;
}

function add(uint256 a, uint256 b) internal pure returns (uint256) {
uint256 c = a + b;
assert(c >= a);
return c;
}
}

/**
* @title Basic token
* @dev Basic version of StandardToken, with no allowances.
*/
contract BasicToken is ERC20Basic {
using SafeMath for uint256;

/**
* @dev transfer token for a specified address
* @param _to The address to transfer to.
* @param _value The amount to be transferred.
*/
function transfer(address _to, uint256 _value) public returns (bool) {
require(_value <= balances[msg.sender]);

// SafeMath.sub will throw if there is not enough balance.
balances[msg.sender] = balances[msg.sender].sub(_value);
Transfer(msg.sender, _to, _value);
return true;
}

/**
* @dev Gets the balance of the specified address.
* @param _owner The address to query the the balance of.
* @return An uint256 representing the amount owned by the passed address.
*/
function balanceOf(address _owner) public view returns (uint256 balance) {
return balances[_owner];
}

}

/**
* @title ERC20 interface
* @dev see https://github.com/ethereum/EIPs/issues/20
*/
contract ERC20 is ERC20Basic {
function approve(address spender, uint256 value) public returns (bool);
}

contract StandardToken is ERC20, BasicToken {

/**
* @dev Transfer tokens from one address to another
* @param _from address The address which you want to send tokens from
* @param _to address The address which you want to transfer to
* @param _value uint256 the amount of tokens to be transferred
*/
require(_value <= balances[_from]);
require(_value <= allowed[_from][msg.sender]);

balances[_from] = balances[_from].sub(_value);
allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
Transfer(_from, _to, _value);
return true;
}

/**
* @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
*
* Beware that changing an allowance with this method brings the risk that someone may use both the old
* and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
* race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
* https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
* @param _spender The address which will spend the funds.
* @param _value The amount of tokens to be spent.
*/
function approve(address _spender, uint256 _value) public returns (bool) {
allowed[msg.sender][_spender] = _value;
Approval(msg.sender, _spender, _value);
return true;
}

/**
* @dev Function to check the amount of tokens that an owner allowed to a spender.
* @return A uint256 specifying the amount of tokens still available for the spender.
*/
return allowed[_owner][_spender];
}

/**
* approve should be called when allowed[_spender] == 0. To increment
* allowed value is better to use this function to avoid 2 calls (and wait until
* the first transaction is mined)
* From MonolithDAO Token.sol
*/
Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
return true;
}

function decreaseApproval(address _spender, uint _subtractedValue) public returns (bool) {
uint oldValue = allowed[msg.sender][_spender];
if (_subtractedValue > oldValue) {
allowed[msg.sender][_spender] = 0;
} else {
allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
}
Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
return true;
}

}

contract TOKKA is StandardToken {
string public name = "TOKKA";
string public symbol = "TOK";
uint256 public decimals = 18;

uint256 constant INITIAL_SUPPLY = 18000000 * 10**18;
function StandardToken() public {
balances[msg.sender] = INITIAL_SUPPLY;
}
}

/**
* @title Crowdsale
* @dev Crowdsale is a base contract for managing a token crowdsale.
* Crowdsales have a start and end timestamps, where investors can make
* token purchases and the crowdsale will assign them tokens based
* on a token per ETH rate. Funds collected are forwarded to a wallet
* as they arrive.
*/

contract Crowdsale {
using SafeMath for uint256;

// The token being sold
TOKKA public token;

// start and end timestamps where investments are allowed (both inclusive)
uint256 public startTime;
uint256 public endTime;

// address where funds are collected

// how many token units a buyer gets per wei
uint256 public rate;

// amount of raised money in wei
uint256 public weiRaised;

/**
* event for token purchase logging
* @param purchaser who paid for the tokens
* @param beneficiary who got the tokens
* @param value weis paid for purchase
* @param amount amount of tokens purchased
*/

function Crowdsale(uint256 _startTime, uint256 _endTime, uint256 _rate, address _wallet) public {
require(_startTime >= now);
require(_endTime >= _startTime);
require(_rate > 0);

startTime = _startTime;
endTime = _endTime;
rate = _rate;
wallet = _wallet;
token = createTokenContract();
}

// creates the token to be sold.
// override this method to have crowdsale of a specific mintable token.
function createTokenContract() internal returns (TOKKA) {
return new TOKKA();
}

// fallback function can be used to buy tokens
function () external payable {
}

// low level token purchase function
require(validPurchase());

uint256 weiAmount = msg.value;

// calculate token amount to be created
uint256 tokens = weiAmount.mul(rate);

// update state

// transfer tokens purchased
ERC20(token).transfer(this, tokens);

TokenPurchase(msg.sender, beneficiary, weiAmount, tokens);

forwardFunds();
}

function forwardFunds() internal {
wallet.transfer(msg.value);
}

function validPurchase() internal view returns (bool) {
bool withinPeriod = now >= startTime && now <= endTime;
bool nonZeroPurchase = msg.value != 0;
return withinPeriod && nonZeroPurchase;
}

function hasEnded() public view returns (bool) {
return now > endTime;
}

}
``````