ActionContext.getContext() returns null using Session in Struts 2

I am trying to use Session in Struts 2:

private Map<String, Object> session = ActionContext.getContext().getSession();

But I'm getting this error:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke 
"com.opensymphony.xwork2.ActionContext.getSession()" because the return value of 
"com.opensymphony.xwork2.ActionContext.getContext()" is null

I tried to use SessionMap of Struts but still got the same error message. I have tried changing the struts-core libraries and also tried a few solutions from Google but still doesn't solve this issue.

ActionContext is a container placeholder for the action invocation, more detailed explanation is here.

There's a lot of reasons when an action context could be null. You are starting another thread, or modify existing one, or it's just not created yet.

To get a SessionMap there's another approach. Let your action class to implement SessionAware and make sure you have configured a default interceptor stack for the action. After that the session map should be injected to the action instance. More about it here.