Sorry, there was a miscommunication, the placements were not random. I put them after the lines that opened the preferred directory, the folder containing the solutions, getting the selected file, and creating the result file. The reasoning for each placement is as follows:

I feel that there should be a return if null after opening the preferred directory because there is a default preferred directory and it being null is an error

I put a return if null after opening the folder because that solves the main problem, clicking cancel on the folder browser after clicking the batch grader button

I put a return if null after opening the selected file because no operations can be done on a file that is null, leading to possible bugs in the future

I put a return if null after creating the result file because the file might not get created if the program doesn't have write permissions in a certain folder, which is an error. I understand that this probably should be followed with an error message to the user, and I will open an issue for this as I don't know the specifics of how exactly to do that.

Lastly, I don't understand how I would implement try-catch blocks for this problem given my very limited experience with the language. If you have any more details or pointers to tutorials I will gladly follow them, but all tutorials I have found so far catch any and all exceptions, which is probably bad practice because the errors can be anything

I will fix the checkstyle errors, thank you for pointing those out to me. It compiled fine for me on my laptop, so I assumed it would be fine to create a pull request

EDIT: Upon examining other places where clicking cancel is handled gracefully, the same method is used in multiple other places

I'll approve these changes since after looking at them again they're good enough to get the issue done.

For catching specific exceptions, simply specify the exception you want to catch in your catch block.

try
{
    // do something
}
catch (SpecificException see)
{
   // do something with se
}
catch (Exception e)
{
   // do something with e
}