Applescript: File remains "open" even if trashed and gone

Question

re: simple text file written in AS alone, outside Finder:
1. I write to a file like this:
  set thisFile to open for access aFilePath with write permission
  — do stuff
  close access thisFile
As long as "do stuff" executes OK., the close access is hit and I can use the file over and over. I have no problems with this script. But:
2. If “do stuff” hits an error, it leaves me with an unusable file. Even when the file is trashed and gone (Secure Empty Trash), if I ever try to use the same file name, AS will first create the file, but immediately pop an error that that file is already open before I can do anything else. The system is remembering the file name and flagging it as open (apparently).
3. So I try to close it:
      set thisFile to file aFilePath
      close access thisFile
and I get no error on the first line but a  “ ... [correct path to "thisFile"] cannot be made into type file" error on the second line.
Question 1: If "thisFile" is a file on the first line, then why not on the second? Why does "close access" which works in item 1 above, fail here?
So any new file name works as long as "doStuff" works, but I can't use any file name that didn't close the first time through. I cannot seem to close the file after the fact, even when it's been trashed.
Question 2: how do I tell the system that this file is not open or does not exist?
Great thanks for your help!

Answer 1

I'm guessing that this is happening to you while running the script in the script editor.
When a script is run in the editor it is the editor that actually has the file open and if the script exits without closing the file the editor gets hung.  Any open files are automatically closed when the application exits but as it is the editor that has the file open the file is not closed when the script exits only when the editor exits. So one work around is to close the editor and restart it.
However the correct way to handle this is to the file work in a try on error block. See Introduction to AppleScript Language Guide specifically the section Close access, the command used to close an open file, for more details.
The basic idea
set aFile to choose file
set fp to open for access aFile
try
  --file reading and writing here
on error e number n
  --deal with errors here and don't resignal
end try
close access fp