Fix crash when adding new ODS file

Crash was caused by async task callback firing when
Fragment is not attached to activity.

Check if fragment is attached before invoking
callback logic.

Fixes: #5020

Signed-off-by: Chris Narkiewicz <hello@ezaquarii.com>

src/main/java/com/owncloud/android/ui/dialog/ChooseTemplateDialogFragment.java

@@ -235,9 +235,9 @@ public class ChooseTemplateDialogFragment extends DialogFragment implements Dial
 
         @Override
         protected void onPostExecute(String url) {
-            ChooseTemplateDialogFragment fragment = chooseTemplateDialogFragmentWeakReference.get();
+            final ChooseTemplateDialogFragment fragment = chooseTemplateDialogFragmentWeakReference.get();
 
-            if (fragment != null) {
+            if (fragment != null && fragment.isAdded()) {
                 if (url.isEmpty()) {
                     DisplayUtils.showSnackMessage(fragment.listView, "Error creating file from template");
                 } else {
@@ -247,7 +247,6 @@ public class ChooseTemplateDialogFragment extends DialogFragment implements Dial
                     collaboraWebViewIntent.putExtra(ExternalSiteWebView.EXTRA_SHOW_SIDEBAR, false);
                     collaboraWebViewIntent.putExtra(ExternalSiteWebView.EXTRA_TEMPLATE, Parcels.wrap(template));
                     fragment.startActivity(collaboraWebViewIntent);
-
                     fragment.dismiss();
                 }
             } else {