flowable rest api start loan request thrown NPE

**Describe the bug**
An NPE occurs when a process is initiated through the REST API.

**Expected behavior**
I followed the tutorial in the link below and started the process using Postman.
https://www.flowable.com/open-source/docs/bpmn/ch10-JPA#advanced-example-using-spring-beans-and-jpa

**Code**
rest api :
`POST` `/runtime/process-instances`

body:
```json
{
"name": "Xiazunyang-LoanRequest-2022/04/28-1",
"processDefinitionKey": "LoanRequestProcess",
"startFormVariables": [
{
"name": "customerName",
"type": "string",
"value": "Xiazunyang"
},
{
"name": "amount",
"type": "long",
"value": 1200
}
]
}
```

the exceptions:

The code for the NPE to occur:
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