NullPointerException when serializing KieSession with a Timer

When running test [1] in a loop, it fails on NullPointerException with the following stacktrace [2]. It is better reproducible using short event expiration (e.g 1ms).

[1] https://github.com/kiegroup/drools/blob/4ce8431e43a3bcd55f367b247d9a4766a11f1b5a/drools-test-coverage/test-compiler-integration/src/test/java/org/drools/compiler/integrationtests/CepEspTest.java#L2912
[2] https://gist.github.com/baldimir/dff726297026f15da9181df2bf4c79fb