I have also seen this while trying to test other behaviour for the conditional buildstep plugin.
However in this case I do not see the exception as this is consumed by the conditional buildstep Runner.
however I do see the the following
"ERROR: Build aborted. No projects to trigger. Check your configuration!"
Looking at the code it points to the function at hudson.plugins.parameterizedtrigger.BuildTriggerConfig.getProjectList(BuildTriggerConfig.java:132)
returning an empty list.
This is called as
List<AbstractProject> projectList = config.getProjectList(build.getProject().getParent(),env);
However when this is a Matrix project
build is a MatrixBuild, and get Project gets the MatrixConfiguration and the parent of that is the MatrixProject which implements the ItemGroup<MatrixConfiguration>
So when Items.fromNameList(context, projectNames.toString(), AbstractProject.class) is called in the context is the MatrixProject so tries to get the project from that.
public List<AbstractProject> getProjectList(ItemGroup context, EnvVars env)
Possible solution would be to do
which should give the topbuild item in the tree