java.lang.NullPointerException空指针错误!在线等!

package LoginPageBean;

import java.util.List;
import javax.faces.application.Application;
import javax.faces.component.html.HtmlCommandButton;
import javax.faces.component.html.HtmlPanelGrid;
import javax.faces.context.FacesContext;

public class LoginBean {

private String username;
private String password;
private String loginmode;
private String ButtonArray[];
private HtmlPanelGrid ButtonGrid;

public String[] getButtonArray() {
return ButtonArray;
}
public void setButtonArray(String[] buttonArray) {
this.ButtonArray = buttonArray;
}
public HtmlPanelGrid getButtonGrid() {
return ButtonGrid;
}
public void setButtonGrid(HtmlPanelGrid buttonGrid) {
this.ButtonGrid = buttonGrid;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;