Basically the issue from #528, now as its own issue.

Error can be found the following way:

However I have found a way to reproduce the bug, by trying to the following code:

interface addInterface {
    RequestResponse: 
    add(int)(int)
}

service bug {

    inputPort httpPort {
        Location: "socket://localhost:8080" 
        protocol: http {.format = json}
        interfaces: addInterface
    }

    main{

        [add(req)(res){
            res = req++
        }]

    }

}

This would run if you delete the {.format = json} part.

The stacktrace from running with --stacktrace is:

java.lang.NullPointerException: Cannot invoke "jolie.State.root()" because the return value of "jolie.ExecutionThread.getState()" is null
        at jolie.runtime.VariablePath.getRootValue(VariablePath.java:129)
        at jolie.runtime.VariablePath.getValueOrNull(VariablePath.java:243)
        at jolie.runtime.VariablePath.evaluate(VariablePath.java:444)
        at jolie.runtime.expression.InlineTreeExpression$AssignmentOperation.run(InlineTreeExpression.java:90)
        at jolie.runtime.expression.InlineTreeExpression.evaluate(InlineTreeExpression.java:143)
        at jolie.OOITBuilder.visit(OOITBuilder.java:509)
        at jolie.lang.parse.UnitOLVisitor.visit(UnitOLVisitor.java:429)
        at jolie.lang.parse.UnitOLVisitor.visit(UnitOLVisitor.java:92)
        at jolie.lang.parse.ast.InputPortInfo.accept(InputPortInfo.java:85)
        at jolie.lang.parse.ast.OLSyntaxNode.accept(OLSyntaxNode.java:47)
        at jolie.OOITBuilder.visit(OOITBuilder.java:615)
        at jolie.OOITBuilder.build(OOITBuilder.java:216)
        at jolie.Interpreter.buildOOIT(Interpreter.java:1239)
        at jolie.Interpreter.init(Interpreter.java:996)
        at jolie.Interpreter.run(Interpreter.java:1079)
        at jolie.Jolie.main(Jolie.java:86)

Originally posted by @Niels-Erik in #528