don't use GetTitle. It depends on windows settings whether extension is visible of not. Besides title can be modified by another add-in.
Use swModel.GetPathName and check for empty string => new not saved file.
When you're uncertain as to what is causing a problem, it is best to create a separate macro that contains only the code necessary to reproduce the problem. If you still can't figure out what the problem is, post that code here. Ivana offered an answer but its speculation because we can't actually see your code or test it on our end.
I have seen the code, but you are right it has nothing to do with GetTitle.
GetTitle code could crash when known file extensions are hidden and filename shorter is then 7 characters.
DefeaturedFileName = swModel.GetTitle DefeaturedFileNameLessExt = Left(DefeaturedFileName, Len(DefeaturedFileName) - 7)
Even DefeaturedFileName as DefeaturedFileNameLessExt are not used anywhere.
I get error 53 in this line, because file sometimes doesn't exists
It should be created here, but it is not, even if boolstatus is true
boolstatus = swModel.Extension.SaveDefeaturedFile(DefeaturedPathName)