Answer by sanket kumar for userinfo.getuserid() returning null in test class

You are passing null manually in your code. See below:-

user  u = handler.createUser(null, sampleData);

This is why you are getting the issue. Here you have done correctly:-

kc_RegHandler authP = new kc_RegHandler();
User temp = authP.createUser(UserInfo.getUserId(), dataX);