FileDetailFragment: prevent NullPointerException if fileDetailSharingFragment is called too soon

Signed-off-by: Álvaro Brey Vilas <alvaro.brey@nextcloud.com>

src/main/java/com/owncloud/android/ui/fragment/FileDetailFragment.java

@@ -169,6 +169,9 @@ public class FileDetailFragment extends FileFragment implements OnClickListener,
      * @return reference to the {@link FileDetailSharingFragment}
      */
     public FileDetailSharingFragment getFileDetailSharingFragment() {
+        if (binding == null) {
+            return null;
+        }
         return ((FileDetailTabAdapter) binding.pager.getAdapter()).getFileDetailSharingFragment();
     }