refactor: fix warnings

lib/widgets/focus/focus_indicator.dart

@@ -136,10 +136,7 @@ class _FocusableWrapperState extends State<FocusableWrapper> {
   }
 
   KeyEventResult _handleKeyEvent(FocusNode node, KeyEvent event) {
-    if (widget.onKeyEvent != null) {
-      return widget.onKeyEvent!(node, event);
-    }
-    return KeyEventResult.ignored;
+    return widget.onKeyEvent?.call(node, event) ?? KeyEventResult.ignored;
   }
 
   @override

lib/widgets/playlist_item_card.dart

@@ -1,5 +1,4 @@
 import 'package:flutter/material.dart';
-import 'package:flutter/services.dart';
 import 'package:cached_network_image/cached_network_image.dart';
 import '../client/plex_client.dart';
 import '../mixins/keyboard_long_press_mixin.dart';