when combining 2 or more open files, when clicking to close one of them, the other will close too.

Steps to reproduce:
1. open 2 pdf files
2. 'combine files into a single PDF' (combine open files) -> you will have file1, file2, Binder1
3. click X to close file1 ->
-> file1 and file2 will both close.
Expected:
only file1 closed.