Wildfly 12.0.0 Final Delay in Getting the Value Attributes of the Session Variables (Result in Null Pointer Exception)

I have two Servlets Book and BookPreview the attribute is set in Book Servlet as follow:

Book.java
TestBean testBean=null;
if(session.getAttribute("testBean")!=null)
testBean = (TestBean)session.getAttribute("testBean");
else{
testBean=new TestBean();
session.setAttribute("testBean",testBean);
}
testBean.setAmount("10");

response.sendRedirect("BookPreview");

The session attribute can be retrieved and the page is loaded normally and the Amount value is displayed however when submitting the form (Post Action in BookPreview.java) Null Pointer exception is generated.

BookPreview.java

TestBean testBean = (TestBean)session.getAttribute("testBean");
String amount = testBean.getAmount; //Null pointer exception when submitting the form

<form method="Post" action="BookPreview">

</form>

TestBean.java

public class TestBean implements java.io.Serializable {

private static final long serialVersionUID = 1L;
private String amount;

public String getAmount()

public void setAmount(String amount)

}

Kindly advice how to fix the problem at the level of the server configuration.

PS: I am not getting the exception when deploying the same application under other J EE application servers (Tomcat....)

Thanks and Best Regards