Question for method to fix errors when making flat image(Export) in ImageJ

Hello, all!
I’ve tried to export my stitched images to make flat image(tried to make tiff format file). Raw images are int8 type image. I tried export>make flat image. Then I’ve got error message in log.

ERROR:
java.lang.NullPointerException
at ini.trakem2.persistence.Loader.getFlatImage(Loader.java:2763)
at ini.trakem2.persistence.Loader.getFlatImage(Loader.java:2754)
at ini.trakem2.persistence.Loader$6.run(Loader.java:2691)
at java.lang.Thread.run(Thread.java:748)

What is the problem and How can I fix it?