Caused: java.io.NotSerializableException: groovy.json.JsonBuilder

I am facing this error Caused: java.io.NotSerializableException: groovy.json.JsonBuilder while building a jenkins pipeline. However the same code is working in jenkins scriptler. Should I do any changes on this code to write a Json file through jenkins pipeline

def vars = ["a": 1,"b": 2,"c": 3,"d": 4,]
def json = new groovy.json.JsonBuilder()
def final_out = json "sample": vars
json "file": final_out
def file = new File("/tmp/sample.json")                                   
file.write(groovy.json.JsonOutput.prettyPrint(json.toString()))

Sample output looks like:

{
    "file": {
        "sample": {
             "a": 1,
             "b": 2,
             "c": 3,
             "d": 4
        }
    }
}