Hey man did you solve this issue? I'm facing the exact NPE with ExecuteAndWaitInterceptor. I've created a custom interceptor stack excluding this one, but with no results.
Then I've introduced several libraries which are required by Struts 6.4.0 like Caffeine, OGNL 3.3.4, javassist 3.29.0-GA, but I get the same result.
I'm kind of stuck and this issue drives me crazy, any suggestion is much appreciated!