Fix compilation error for GalleryFastScrollViewHelper

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

app/src/main/java/com/owncloud/android/ui/fragment/util/GalleryFastScrollViewHelper.kt

@@ -238,7 +238,7 @@ class GalleryFastScrollViewHelper(
         return if (position == RecyclerView.NO_POSITION) {
             null
         } else {
-            popupTextProvider.getPopupText(position)
+            popupTextProvider.getPopupText(position).toString()
         }
     }