So, a particular (large) fo file that I tried to convert to PDF crashed FOP. I looked into the issue and discovered that for whatever reason, a child LayoutContext was null, and when FOP tried to set the flags for the child context, it would fail with a null-pointer exception. I fixed the issue by putting in some null-checking in the InlineStackingLayoutManager.java file. I'm not sure if that is the correct approach but it certainly enabled me to to produce the PDF using FOP, which is a start :)
Created attachment 5961 [details] A patch that fixes the issue
move to normal priority, pending further action
increase priority for bugs with a patch