migrate selected_files.dart to null safety

lib/models/selected_files.dart

@@ -1,5 +1,4 @@
-// @dart=2.9
-
+import 'package:collection/collection.dart' show IterableExtension;
 import 'package:flutter/foundation.dart';
 import 'package:photos/models/file.dart';
 
@@ -11,9 +10,8 @@ class SelectedFiles extends ChangeNotifier {
     // To handle the cases, where the file might have changed due to upload
     // or any other update, using file.generatedID to track if this file was already
     // selected or not
-    final File alreadySelected = files.firstWhere(
+    final File? alreadySelected = files.firstWhereOrNull(
       (element) => element.generatedID == file.generatedID,
-      orElse: () => null,
     );
     if (alreadySelected != null) {
       files.remove(alreadySelected);
@@ -26,17 +24,15 @@ class SelectedFiles extends ChangeNotifier {
   }
 
   bool isFileSelected(File file) {
-    final File alreadySelected = files.firstWhere(
+    final File? alreadySelected = files.firstWhereOrNull(
       (element) => element.generatedID == file.generatedID,
-      orElse: () => null,
     );
     return alreadySelected != null;
   }
 
   bool isPartOfLastSection(File file) {
-    final File alreadySelected = lastSelections.firstWhere(
+    final File? alreadySelected = lastSelections.firstWhereOrNull(
       (element) => element.generatedID == file.generatedID,
-      orElse: () => null,
     );
     return alreadySelected != null;
   }