Fix NPE when navigating out of PreviewMediaFragment during video loading

Although original error has been fixed by migration to Exo player,
there is still a potential NPE when fragment went through
onDestroyView().

Fixes #8620

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

src/main/java/com/owncloud/android/ui/preview/PreviewMediaFragment.java

@@ -507,7 +507,7 @@ public class PreviewMediaFragment extends FileFragment implements OnTouchListene
         protected void onPostExecute(Uri uri) {
             final PreviewMediaFragment previewMediaFragment = previewMediaFragmentWeakReference.get();
             final Context context = previewMediaFragment != null ? previewMediaFragment.getContext() : null;
-            if (previewMediaFragment != null && context != null) {
+            if (previewMediaFragment != null && previewMediaFragment.binding != null && context != null) {
                 if (uri != null) {
                     previewMediaFragment.videoUri = uri;