Fixed code smells

lib/pages/app.dart

@@ -32,6 +32,7 @@ class _AppPageState extends State<AppPage> {
     getUpdate(String id) {
       appsProvider.checkUpdate(id).catchError((e) {
         showError(e, context);
+        return null;
       });
     }
 

lib/pages/apps.dart

@@ -71,6 +71,7 @@ class AppsPageState extends State<AppsPage> {
       });
       return appsProvider.checkUpdates().catchError((e) {
         showError(e, context);
+        return <App>[];
       }).whenComplete(() {
         setState(() {
           refreshingSince = null;
@@ -379,6 +380,7 @@ class AppsPageState extends State<AppsPage> {
                       [listedApps[appIndex].app.id],
                       globalNavigatorKey.currentContext).catchError((e) {
                     showError(e, context);
+                    return <String>[];
                   });
                 },
           icon: Icon(
@@ -681,6 +683,7 @@ class AppsPageState extends State<AppsPage> {
                           settingsProvider: settingsProvider)
                       .catchError((e) {
                     showError(e, context);
+                    return <String>[];
                   });
                 }
               });

lib/pages/import_export.dart

@@ -323,8 +323,8 @@ class _ImportExportPageState extends State<ImportExportPage> {
                         ],
                       ),
                       if (importInProgress)
-                        Column(
-                          children: const [
+                        const Column(
+                          children: [
                             SizedBox(
                               height: 14,
                             ),