How to fix error: type 'Null' is not a subtype of type '() => void'

I have an admin sign in page where admins can add an id and password to gain access to the Admin area. There is an admin collection in the Firebase Database that stores the id and password. The admin collection is separate from the user sign in page and separate from the user collection which uses Firebase Authentication before allowing access. The user access continues to work correctly. When I fill in the two input boxes on the admin sign in screen and click the button to gain access my error dialog message appears indicating that there isn't any data in the two input fields even though there is data. If I do nothing to the code and then hot reload and click the button again I am able to access the admin but I get the following error message in the console.

The following _TypeError was thrown building ShoppingAdminSignInPage(dirty, dependencies: [_LocalizationsScope-[GlobalKey#2c797]], state: _ShoppingAdminSignInPageState#e3b3d): type 'Null' is not a subtype of type '() => void'

I have obviously written something or several things incorrectly in my code. It appears the error is in the ShoppingAdminSignInButton. Thank you in advance for any help.

class ShoppingAdminSignInPage extends StatefulWidget {
  const ShoppingAdminSignInPage({Key? key}) : super(key: key);

  @override
  State<ShoppingAdminSignInPage> createState() =>
      _ShoppingAdminSignInPageState();
}

class _ShoppingAdminSignInPageState extends State<ShoppingAdminSignInPage> {
  final GlobalKey<FormState> _formKey = GlobalKey<FormState>();
  final TextEditingController _adminIDController = TextEditingController();
  final TextEditingController _passwordController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return AdaptiveLayoutScaffold(
      appBar: const ShoppingAdminSignInPageAppBar(),
      landscapeBodyWidget: Container(),
      portraitBodyWidget: SingleChildScrollView(
        child: Padding(
          padding: const EdgeInsets.symmetric(horizontal: 16.0),
          child: Column(
            mainAxisSize: MainAxisSize.max,
            children: [
              const ShoppingAdminSignInHeader(),
              Form(
                key: _formKey,
                child: Column(
                  children: [
                    const SizedBox(
                      height: 50.0,
                    ),
                    AdminSignInTextField(
                      controller: _adminIDController,
                      labelText: TextFieldLabel.adminID,
                      prefixIcon: Icons.person,
                      textInputAction: TextInputAction.next,
                    ),
                    AdminSignInTextField(
                      controller: _passwordController,
                      labelText: TextFieldLabel.password,
                      prefixIcon: Icons.password,
                      textInputAction: TextInputAction.done,
                    ),
                    ShoppingAdminSignInButton(
                      onPressed: _adminIDController.text.isNotEmpty &&
                              _passwordController.text.isNotEmpty
                          ? logInAdmin()
                          : () => showDialog(
                              context: context,
                              builder: (ctx) {
                                return const ErrorAlertDialog(
                                  message: DialogString.addAdminIDAndPassword,
                                );
                              }),
                    ),
                    const NotAnAdminButton(),
                  ],
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }

  logInAdmin() {
    FirebaseFirestore.instance.collection('admins').get().then((snapshot) {
      snapshot.docs.forEach((result) {
        if (result.data()['id'] != _adminIDController.text.trim()) {
          SnackBarUtil.showSnackBar(
            context,
            SnackBarString.idNotCorrect,
          );
        } else if (result.data()['password'] !=
            _passwordController.text.trim()) {
          SnackBarUtil.showSnackBar(
            context,
            SnackBarString.passwordNotCorrect,
          );
        } else {
          SnackBarUtil.showSnackBar(
            context,
            'Welcome ${result.data()['name']}',
          );
          setState(() {
            _adminIDController.text = '';
            _passwordController.text = '';
          });
          Navigator.pushReplacement(
            context,
            MaterialPageRoute(
              builder: (context) => const UploadItemsPage(),
            ),
          );
        }
      });
    });
  }
}