fix: Avoid throwing on folder previews as there are none

Signed-off-by: Julius Härtl <jus@bitgrid.net>

apps/files/src/components/FileEntry.vue

@@ -395,6 +395,10 @@ export default Vue.extend({
 			return this.userConfig.crop_image_previews
 		},
 		previewUrl() {
+			if (this.source.type === 'folder') {
+				return null
+			}
+
 			try {
 				const previewUrl = this.source.attributes.previewUrl
 					|| generateUrl('/core/preview?fileId={fileid}', {