如何注销用户firebase？

如何解决如何注销用户firebase？

我有一个 on press 方法可以将用户注销。当用户退出时，他无法导航到注册页面，但他可以重新登录。我应该说我不使用流。 这是我的登出 onpressed 方法：

 onpressed:()async{
                          FirebaseAuth.instance.signOut().then((value) => Navigator.pushNamedAndRemoveUntil(context,LoginScreen.route,(route) => false));

                          },

这是错误：

The method 'call' was called on null.
Receiver: null
Tried calling: call()


When the exception was thrown,this was the stack: 
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:54:5)
#1      _LoginScreenState._buildSignupBtn.<anonymous closure> (package:project/authenticate/signin.dart:186:26)
#2      GestureRecognizer.invokeCallback (package:Flutter/src/gestures/recognizer.dart:182:24)
#3      TapGestureRecognizer.handleTapUp (package:Flutter/src/gestures/tap.dart:607:11)
#4      BaseTapGestureRecognizer._checkUp (package:Flutter/src/gestures/tap.dart:296:5)
...
Handler: "onTap"
Recognizer: TapGestureRecognizer#37170
  debugOwner: GestureDetector
  state: ready
  won arena
  finalPosition: Offset(301.1,574.8)
  finalLocalPosition: Offset(222.4,18.8)
  button: 1
  sent tap down

这就是我可以在登录页面和注册页面之间切换的toogleview：

Widget _buildSignupBtn() {
    return GestureDetector(
      onTap: () {
        widget.toogleView();
      },//onTap: () => print('Sign Up Button pressed'),child: RichText(
        text: TextSpan(
          children: [
            TextSpan(
              text: 'Don\'t have an Account? ',style: TextStyle(
                color: Colors.white,fontSize: 18.0,fontWeight: FontWeight.w400,),TextSpan(
              text: 'Sign Up',fontWeight: FontWeight.bold,],);
  }