Android: prevent NPE due to lack of permissions on a directory

Change-Id: Ia2f4c04ad4c524e8b72870acb0735793624f0916
Reviewed-on: https://gerrit.libreoffice.org/13278Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>

android/experimental/LOAndroid3/src/java/org/libreoffice/ui/FolderIconView.java

@@ -83,6 +83,10 @@ public class FolderIconView extends View{
         if( dir == null )
             return;//TODO
         File[] contents = dir.listFiles();//TODO consider filtering thumbs to match grid.
+        if( contents == null )
+            // dir is not a directory,
+            // or user does not have permissions to read it
+            return;
         Stack<Bitmap> thumbs = new Stack<Bitmap>();
         BitmapFactory factory = new BitmapFactory();
         for( File file : contents ){