What is the difference between `var x = null` and `var x = _`?

initializing.scala

package initializing
class WithNull { var x: Any = null }
class WithUnderscore { var x: Any = _ }

selenium.scala

class PageObject(driver: WebDriver) {
  WebDriverFactory.initElementsWithAjaxSupport(this,driver)  
}

class FooPage(driver:WebDriver) extends PageObject(driver) {
  @FindBy(name="bar")
  private var barButton:WebElement = null // this is what all about
}

WithNull.class

public initializing.WithNull();
  Code:
   0:  aload_0
   1:	invokespecial	#19; //Method java/lang/Object."<init>":()V
   4:	aload_0
   5:	aconst_null
   6:	pop
   7:	aconst_null
   8:	putfield	#11; //Field x:Ljava/lang/Object;
   11:	return

}

WithUnderscore.class

public initializing.WithUnderscore();
  Code:
   0:  aload_0
   1:	invokespecial	#19; //Method java/lang/Object."<init>":()V
   4:	return

}