서버 연동은 했고 

로그인 버튼을 누르면 이런 오류가 납니다!

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot read field "db" because "this.this$0.o" is null
    at Setting.MainFrame$ButtonListener.actionPerformed(MainFrame.java:680)
    at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1972)
    at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2313)
    at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
    at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
    at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:279)
    at java.desktop/java.awt.Component.processMouseEvent(Component.java:6626)
    at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3389)
    at java.desktop/java.awt.Component.processEvent(Component.java:6391)
    at java.desktop/java.awt.Container.processEvent(Container.java:2266)
    at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:5001)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2324)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4833)
    at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4948)
    at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4575)
    at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4516)
    at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2310)
    at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2780)
    at java.desktop/java.awt.Component.dispatchEvent(Component.java:4833)
    at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:773)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:722)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:716)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:399)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:97)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:746)
    at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:744)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:399)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
    at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:743)
    at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

로그인 쪽 코드는

class ButtonListener implements ActionListener{
		@Override
		public void actionPerformed(ActionEvent e) {
			JButton b = (JButton)e.getSource();
			
			/* TextField에 입력된 아이디와 비밀번호를 변수에 초기화 */
			String uid = id.getText();
			String upass = "";
			for(int i=0; i<pw.getPassword().length; i++) {
				upass = upass + pw.getPassword()[i];
			}
			/* 로그인 버튼 이벤트 */
			 if(b.getText().equals("로그인")) {
				if(uid.equals("") || upass.equals("")) {
					JOptionPane.showMessageDialog(null, "아이디와 비밀번호 모두 입력해주세요", "로그인 실패", JOptionPane.ERROR_MESSAGE);
					System.out.println("로그인 실패 > 로그인 정보 미입력");
				}
				
				else if(uid != null && upass != null) {
					//여기서부터 오류 시작인듯 시발. 
					
					if(o.db.logincheck(uid, upass)) {	//이 부분이 데이터베이스에 접속해 로그인 정보를 확인하는 부분이다. 
						
						System.out.println("로그인 성공");
						JOptionPane.showMessageDialog(null, "로그인에 성공하였습니다");
					} else {
						System.out.println("로그인 실패 > 로그인 정보 불일치");
						JOptionPane.showMessageDialog(null, "로그인에 실패하였습니다");
					}
				}
			}
		}
	}

// database 클래스

boolean logincheck(String _i, String _p) {
		boolean flag = false;
		
		String id = _i;
		String pw = _p;
		
		try {
			String checkingStr = "SELECT password FROM mainsh.member WHERE id='" + id + "'";
			ResultSet result = stmt.executeQuery(checkingStr);
			
			int count = 0;
			while(result.next()) {
				if(pw.equals(result.getString("password"))) {
					flag = true;
					System.out.println("로그인 성공");
				}
				
				else {
					flag = false;
					System.out.println("로그인 실패");
				}
				count++;
			}
		} catch(Exception e) {
			flag = false;
			System.out.println("로그인 실패 > " + e.toString());
		}
		
		return flag;
	}