蓝牙扫描过程中，操作关掉蓝牙，后面出现崩溃

[shenh1986]

日志如下：
W/BluetoothAdapter(18861): getBluetoothLeScanner() ble not available
D/AndroidRuntime(18861): Shutting down VM
E/AndroidRuntime(18861): FATAL EXCEPTION: main
E/AndroidRuntime(18861): java.lang.NullPointerException: Attempt to invoke virtual method 'void android.bluetooth.le.BluetoothLeScanner.stopScan(android.bluetooth.le.ScanCallback)' on a null object reference
E/AndroidRuntime(18861): at gao.xiaolei.flutter_blue_elves.FlutterBlueElvesPlugin.lambda$getScanTimeoutCallback21$6$FlutterBlueElvesPlugin(FlutterBlueElvesPlugin.java:490)
E/AndroidRuntime(18861): at gao.xiaolei.flutter_blue_elves.-$$Lambda$FlutterBlueElvesPlugin$2V9C3iupvr1z0-5_MvV3wO8BJaw.run(Unknown Source:2)
E/AndroidRuntime(18861): at android.os.Handler.handleCallback(Handler.java:938)
E/AndroidRuntime(18861): at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime(18861): at android.os.Looper.loop(Looper.java:246)
E/AndroidRuntime(18861): at android.app.ActivityThread.main(ActivityThread.java:8653)
E/AndroidRuntime(18861): at java.lang.reflect.Method.invoke(Native Method)
E/AndroidRuntime(18861): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
E/AndroidRuntime(18861): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
I/Process (18861): Sending signal. PID: 18861 SIG: 9

[pineappleoOilPrince]

这个bug是因为蓝牙关闭的情况下定时停止扫描导致的 会在下一个版本修复