Anthony,
sorry - I'm really trying to help you, but you have to answer my questions - otherwise I'll give up...
What is your correct Jenkins version?
Because if you do not have installed the m2-extra-steps plugin (https://wiki.jenkins-ci.org/display/JENKINS/M2+Extra+Steps+Plugin) then your configuration would not be possible with the Jenkins version your telling us...
Maybe a screenshot with some more context around your single step would help too...

Domi,

My correct Jenkins version is as I stated before, 1.424.6.
I attempted installing the m2-extra-steps plugin and was still receiving the null pointer with the multiple steps.
I've attached the single step as requested.

Anthony,
I'm looking into the code and I'm still stuck, the main question which you still have not answered is:

how do you add a 'post build step'? which section? which button? ...
in the version you mention, this just does not exist!

btw. Maven or FreeStyle Job?
so where/how do you add such a post build action? please be more precise and as requested before, add screenshots which explain the whole configuration not just a snippet.

Code changed in jenkins
User: imod
Path:
src/main/java/org/jenkinsci/plugins/conditionalbuildstep/ConditionalBuilder.java
http://jenkins-ci.org/commit/conditional-buildstep-plugin/a13c4d9bc48e48557a76c379560ad9751af15b1d
Log:
[FIXED JENKINS-13112] Adding any post-build step as a build step causes exception

Compare: https://github.com/jenkinsci/conditional-buildstep-plugin/compare/0c533f8...a13c4d9

I think I finally figured you'r usecase - if it does not work, please open the issue again...