Null point exception while running webdriver script

Sometimes simple mistakes will take lot of time in debugging . I can bet on it :P
Today while runing a sample script I got errors like below :

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)

Here is the script with syntax error:

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1. import org.openqa.selenium.WebDriver;  
2. import org.openqa.selenium.firefox.FirefoxDriver;  
3. import org.openqa.selenium.firefox.FirefoxProfile;  
4. import org.testng.annotations.BeforeTest;  
5. import org.testng.annotations.Test;  
6.   
7. public class AutoComplete {  
8.    
9.  WebDriver driver;  
10.    
11.  @BeforeTest  
12.  public void start(){  
13.  WebDriver driver = new FirefoxDriver();  
14.  }  
15.    
16.  @Test  
17.  public void AutoComplete()  
18.  {  
19.  driver.get("http://mythoughts.co.in/");  
20.  driver.manage().window().maximize();  
21.  }  
22.   
23. }  

I just overlooked the above script and thought everything is fine. But the issue got fixed when I remove the "WebDriver" statement from the line 13 and script ran successfully.

Here is the script which ran successfully.

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1. import org.openqa.selenium.WebDriver;  
2. import org.openqa.selenium.firefox.FirefoxDriver;  
3. import org.openqa.selenium.firefox.FirefoxProfile;  
4. import org.testng.annotations.BeforeTest;  
5. import org.testng.annotations.Test;  
6.   
7. public class AutoComplete {  
8.    
9.  WebDriver driver;  
10.    
11.  @BeforeTest  
12.  public void start(){  
13.  driver = new FirefoxDriver();  
14.  }  
15.    
16.  @Test  
17.  public void AutoComplete()  
18.  {  
19.  driver.get("http://mythoughts.co.in/");  
20.  driver.manage().window().maximize();  
21.  }  
22.   
23. }