When creating an application image, the --name option is not strictly required, and jpackage will infer the name from the main class.

If main class is not explicitly given (either by --module/<main-class> for modular app, or --main-class <main-class> for non-modular app), it too can be inferred, either from the --module-path or --main-jar pointing to a jar with a Main-Class manifest attribute.

However, the combination of both of these do not work, and the command:
jpackage create-app-image -o output -p input/mjar1.jar -m me.mymodule
will fail with strange error:
Error: Application output <...>\output already exists.

It seems at the point it is creating the output directory "output/<name>" name is null.