Help needed for Struts 6.6.0 tagnames in UIBean.java

Need expert opinion: I am using Struts 6.6.0.  While clicking a back button
in my application, I am getting below exception in UIBean.java. I have
added all dependencies needed for Struts 6.6.0, as mentioned in release
version notes. Can anyone help to understand what could cause this.? The
page works when first time load, then from next page if I come back, I am
getting below exception:

Caused by: java.lang.NullPointerException: Cannot invoke
"java.util.List.add(Object)" because "tags" is null
at org.apache.struts2.components.UIBean.evaluateParams(UIBean.java:804)
~[struts2-core-6.6.0.jar:6.6.0]


*Thanks & Regards*
*Santu Dhua*