is their a way to key a error message that is thrown from a text box input in flutter for my integration tests

Question

im having some issues with my integration tests in flutter on how i can check if an error pops up when wrong value is input idk how to check for it any help would be much appeciated.

` 

 child: TextFormField(
                        key: Key('UserEntry_btc'),
                        textAlign: TextAlign.center,
                        // inputFormatters: [
                        //   FilteringTextInputFormatter.digitsOnly
                        // ],
                        decoration: InputDecoration(
                          errorText:
                              showError ? guessTextFieldErrorMessage : null,
                          hintText: "Enter USD amount",
                        ),
                        keyboardType: TextInputType.number,
                        controller: UserInput,
                      ),
                    )),
              )),
              Padding(padding: EdgeInsets.all(4)),
              SizedBox(
                  width: 280,
                  height: 45,
                  child: ElevatedButton(
                    key: Key("ConvertButton_btc"),

                    style: ElevatedButton.styleFrom(
                      primary: Color(0xFF4C748B),
                      shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(17),
                      ),
                      elevation: 0.0,
                    ),

                    //Code in OnPressed

                    onPressed: () {
                      USD = UserInput.text;

                      setState(() {
                        visible = false;
                        try {
                          if (UserInput.text.isEmpty ||
                              UserInput.text.contains(RegExp(r'[a-z,A-Z]+$'))) {
                            showError = true;
                            visible = false;
                            guessTextFieldErrorMessage =
                                'Invalid input, must be an number';
                          } else {
                            visible = true;
                            USD =
                                BtcTools.BtcToUsd(double.parse(UserInput.text))
                                    .toStringAsFixed(2);
                            showError = false;
                          }
                        } catch (e) {
                          //If the user is guessing an invalid character return this message
                          guessTextFieldErrorMessage =
                              'Invalid input, must be an number';
                          showError = true;
                          visible = false;
                        }
                      });
                      USD = BtcTools.UsdToBtc(double.parse(UserInput.text))
                          .toString();
                      // USD = BtcTools.UsdToBtc(double.parse(UserInput.text))
                      // .toStringAsFixed(7);
                    },
`

here is how im running some tests i do not know what im needing to call in the test if their is a way.

`test("should give conversion for USD to BTC", () async {
      //expect(await driver.getText(apptitle), 'Currency Converter App');
      await driver.tap(btc_selection_button);
      //expect(await driver.getText(btc_entry), 'Enter USD amount');
      await driver.tap(btc_entry);
      await driver.enterText('-3');
      await driver.waitFor(find.text('-3'));
      await driver.tap(btc_convert_button);
      expect(
          await driver.getText(btc_error), 'Invalid input, must be an number');
      //expect(await driver.getText(btc_result), '0.9941365 BTC');
      await driver.tap(btc_back_button);
    });`

ive tried to double key it but i dont seem to understand how or where to.