There is no PasswordEncoder mapped for the id "null"

造成这个问题的原因具体请参看 

Spring Security 5.0的DelegatingPasswordEncoder详解  

地址：https://blog.youkuaiyun.com/alinyua/article/details/8021950

仔细看。

解决办法就是

@Bean
public  static PasswordEncoder passwordEncoder(){

  DelegatingPasswordEncoder delegatingPasswordEncoder = 
     (DelegatingPasswordEncoder)PasswordEncoderFactories.createDelegatingPasswordEncoder();

//设置defaultPasswordEncoderForMatches为NoOpPasswordEncoder      
delegatingPasswordEncoder.setDefaultPasswordEncoderForMatches(NoOpPasswordEncoder.getInstance());

return  delegatingPasswordEncoder;
}