How to retrive all fields in a table using Graph

There's a way to retrive all the fields in a table?

I need to provide a system where the user can create custom field for a certain table

I tried this but got an unexpected crash

let db : Graph = Graph()
let test = Entity(type: "test", graph: db)
test["field1"] = "test1"
test["field2"] = "test2"
test["field3"] = "test3"

let dict = test.properties
dump(dict.keys) // <- crash here, unexpectedly found nil when bla bla

thank you