Python GDT import script issue?

[phix33]

I can run the following python script in the GUI and does what I expect (opens GDT archive, imports the data types and closes the archive). However, if I run it as preScript in headless, the data types are imported however the GDT archive is left in a 'weird unclosed state' such that when you open the project in the GUI the archive is still associated with the project but it can no longer find the archive. I'm assuming I've missed something here in the script that the GUI is fixing for me - anyone any ideas?

from ghidra.program.model.data import FileDataTypeManager

dtm = currentProgram.getDataTypeManager()
gdt_file = java.io.File("./archive.gdt")
fdtm = FileDataTypeManager.openFileArchive(gdt_file, False)
for dt in fdtm.getAllDataTypes():
    dtm.addDataType(dt, None)