使用IDEA运行时出现HTTP status 500-Internal Server Error 的错误

使用IDEA运行时出现HTTP status 500-Internal Server Error 的错误

Type Exception Report

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

java.lang.NullPointerException
net.servlet.loginServlet.doGet(loginServlet.java:23)
javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
Note The full stack trace of the root cause is available in the server logs.

Apache Tomcat/8.5.34
请问这是什么原因？？