Java报错exception in thread awt-eventqueue-0这个怎么解决呀

public void actionPerformed(ActionEvent e) {

flag=0;

try{

String num,name,gender,address,phone,major;

con=DriverManager.getConnection(“jdbc:mysql://localhost:3306/university?characterEncoding=utf8″,”root”,”123456″);

num=t1.getText().trim();

ResultSet rs=sql.executeQuery(“SELECT * FROM student WHERE id='”+num+”‘”);

if(rs.next()){

name=rs.getString(“name”);

gender=rs.getString(“gender”);

address=rs.getString(“address”);

phone=rs.getString(“phone”);

major=rs.getString(“major”);

t2.setText(name);

t3.setText(gender);

t4.setText(address);

t5.setText(phone);

t6.setText(major);

if(e.getSource()==b2){

String temp=”DELETE FROM student WHERE id='”+num+”‘”;

sql.executeUpdate(temp);

JOptionPane.showMessageDialog(this,”删除成功!”,”提示对话框”,JOptionPane.INFORMATION_MESSAGE);

}

flag=1;

}else{

JOptionPane.showMessageDialog(this,”没有该学生!”,”提示对话框”,JOptionPane.INFORMATION_MESSAGE);

}

con.close();

if(flag==0){t1.setText(“没有该学生”);}

}

catch(SQLException ee){}

}

}

错误

Exception in thread “AWT-EventQueue-0” java.lang.NullPointerException: Cannot invoke “java.sql.Statement.executeQuery(String)” because “this.sql” is null

at GUI.StudentDelete.actionPerformed(StudentDelete.java:90)

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$JavaSecurityA