Publishing area tree XML fails

I'm using FOP 2.4 on ubuntu. I create an area tree XML file on the 
command line with this:

fop -s topic.fo -at area_tree.xml

This works fine. I plan to make some changes and read it in, then 
publish to PDF.

I am trying to read it in and I don't know what to expect, or how to 
publish out to PDF. The docs do not go into this, that I can find. I get 
failure when I run the command below intended to produce a PDF called 
aaa.pdf:

fop -atin area_tree.xml aaa.pdf
[warning] /usr/bin/fop: JVM flavor 'sun' not understood
[ERROR] FOP - Exception <org.apache.fop.apps.FOPException: 
java.util.EmptyStackException
java.util.EmptyStackException>org.apache.fop.apps.FOPException: 
java.util.EmptyStackException
java.util.EmptyStackException
         at 
org.apache.fop.cli.InputHandler.transformTo(InputHandler.java:296)
         at 
org.apache.fop.cli.AreaTreeInputHandler.renderTo(AreaTreeInputHandler.java:75)
         at org.apache.fop.cli.Main.startFOP(Main.java:183)
         at org.apache.fop.cli.Main.main(Main.java:214)
Caused by: java.util.EmptyStackException
         at java.base/java.util.Stack.peek(Stack.java:102)
         at 
org.apache.fop.area.AreaTreeParser$Handler.handleExternallyGeneratedObject(AreaTreeParser.java:1089)
etc.

Any ideas?

Thanks,
Mark