当我尝试在web hive.box（'box\u name'）.listenable（）上使用这个例子时，如果没有键，就会发生错误；

════════ Exception caught by widgets library ═══════════════════════════════════
The following NoSuchMethodError was thrown building FutureBuilder<dynamic>(dirty, state: _FutureBuilderState<dynamic>#566bb):
The method 'toSet' was called on null.
Receiver: null
Tried calling: toSet()

The relevant error-causing widget was
    FutureBuilder<dynamic> 
package:hive_benchmark/sketcpad/sketchpad.dart:109
When the exception was thrown, this was the stack

# 0      Object.noSuchMethod  (dart:core-patch/object_patch.dart:53:5)

# 1      BoxX.listenable

package:hive_flutter/src/box_extensions.dart:11

# 2      _DrawingScreenState.build.<anonymous closure>

package:hive_benchmark/sketcpad/sketchpad.dart:120

# 3      _FutureBuilderState.build

package:flutter/…/widgets/async.dart:732

# 4      StatefulElement.build

package:flutter/…/widgets/framework.dart:4619
...
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