Expected a value of type 'SkDeletable', but got one of type 'Null' any solutions?

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Future uploadBannerImageToDb(url) async { String downloadUrl = await storge.ref(url).getDownloadURL();

if (downloadUrl != null) {
  firestore.collection('slider').add({
    'image': downloadUrl,
  });
}
return downloadUrl;

} /********************/

onPressed: () { progressDialog.show(); _services .uploadBannerImageToDb(_url) .then((downloadUrl) { if (downloadUrl != null) { progressDialog.dismiss(); _services.showMyDialog( title: 'New Banner Image', message: 'Saved Banner Image Successfully', context: context, ); } }); },