Flutter Form controller throws "The method 'validate' was called on null"

Question

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

I don't understand this. I thought maybe the problem was the Form isn't the root element of the class, it's not return Form(child: Column(children: [... So I tried making the Form Widget the root, it stopped the error, but didn't activate the TextFormField validator or save, it just said 'everything fine, move along'.

It's just one field I presently wish to validate. I've looked up other such queries, both the Form widget & the TextFormField have keys, so I'm stuck.

I declare the form key with final _formKeyForDeposit = GlobalKey<FormState>();

And here is the un-cooperative form:

Form(key: _formKeyForDeposit, child:
TextFormField(
    controller: _controllerDefaultDeposit,
    key: Key('defaultLoanDeposit'),
    decoration: InputDecoration(
      //icon: Icon(Icons.location_city),
      labelText: 'Per item deposit',
      hintText: 'Whole numbers',
      suffixIcon: IconButton(
        icon: Icon(Icons.clear),
        onPressed: () {
          _controllerDefaultDeposit.clear();
        },
      ),
    ),
    keyboardType: TextInputType.numberWithOptions(decimal: false, signed: false),
    onSaved: (String amountStr) {
      print("saving deposit");
      user.defaultItemDeposit = int.parse(amountStr.trim());
    },
    validator: (String value) {
      print(LOG + "validator called");
      if(int.tryParse(value.trim()) == null) {
        inputCompletionAlert += "But your default item deposit is not a number, please correct.\n";
        return 'Not a £-- whole number monetary amount';
    }
      if(value == "" || value == "0") {
        print(LOG + 'deposit validator called, should launch Dialog from here');
        inputCompletionAlert += "Would you like to set a default deposit?";
        return "Would you like to set a deposit?";
      }
      return null;
    },
  ),
),

Solution

Have you tried building a custom validator function and then directly calling it from the validator property.

For example :-

Validator (String value) {
 print(LOG + "validator called"); 
if(int.tryParse(value.trim()) == null) {
 inputCompletionAlert += "But your default item deposit is not a number, please correct.\n";
 return 'Not a £-- whole number monetary amount'; 
  } 
}

Comments

[Aman khan Roohaani]

Also what are you using int.tryParse() for??

[Sam]

To detect whether the user hasn't given me a number, it returns null if it can't convert it to a number, at which point I fail validation, whilst avoiding throwing an exception. "tryParse: Parses a string containing a number literal into a number. Like parse except that this function returns null for invalid inputs instead of throwing."