Avoid usage of requireActivity in download callback (#6558)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/488551667048375/task/1211022417452920?focus=true

### Description
After downloading a file we have a callback which shows a snackbar;
there is button on it to open the file. From a stack trace it seems
possible the activity is gone and the use of `requireActivity()` results
in a crash. Switching to avoiding the crash and doing nothing if the
`activity` is null when we try.

Fixes #6557

### Steps to test this PR
- [ ] QA optional
- [ ] If you want, download a file (e.g.,
https://file-examples.com/index.php/sample-images-download/sample-jpg-download/)
and tap on the snackbar to view it once done

Co-authored-by: Craig Russell <[email protected]>

Author: CDRussell

app/src/main/java/com/duckduckgo/app/browser/BrowserTabFragment.kt

@@ -1579,9 +1579,11 @@ class BrowserTabFragment :
         )
             ?.apply {
                 this.setAction(R.string.downloadsDownloadFinishedActionName) {
-                    val result = downloadsFileActions.openFile(requireActivity(), File(command.filePath))
-                    if (!result) {
-                        view.makeSnackbarWithNoBottomInset(getString(R.string.downloadsCannotOpenFileErrorMessage), Snackbar.LENGTH_LONG).show()
+                    activity?.let {
+                        val result = downloadsFileActions.openFile(it, File(command.filePath))
+                        if (!result) {
+                            view.makeSnackbarWithNoBottomInset(getString(R.string.downloadsCannotOpenFileErrorMessage), Snackbar.LENGTH_LONG).show()
+                        }
                     }
                 }
             }