Erro ao executar POST após configuração do EUREKA

Após a configuração do Eureka fui executar o POST http://localhost:8080/compra e obtive o erro abaixo:

{
    "timestamp": "2020-04-14T02:27:30.948+0000",
    "status": 500,
    "error": "Internal Server Error",
    "message": "No message available",
    "trace": "java.lang.NullPointerException\r\n\tat br.com.alura.microservice.loja.service.CompraService.realizaCompra(CompraService.java:23)\r\n\tat br.com.alura.microservice.loja.controller.CompraController.realizaCompra(CompraController.java:21)\r\n\tat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)\r\n\tat sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)\r\n\tat sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)\r\n\tat java.lang.reflect.Method.invoke(Unknown Source)\r\n\tat org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:190)\r\n\tat