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

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;
},
),
),

Answer 1

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';
}
}

Answer 2

This was an incomplete question, this array of ExpansionBoxes messes up the validator:
ExpansionPanelList.radio(initialOpenPanelValue: 2,
children: [
bicyclePanel,
carPanel,
floodPanel,
diyPanel,
surplusPanel,
gardeningPanel,
ballSportsPanel,
snowSportsPanel,
waterSportsPanel,
campingPanel,
backpackingPanel,
circusPanel,
]),
I presume that when _formKeyForDeposit.currentState.validate() is called it heads down into the ExpansionPanelList and can't escape to trigger the validator of TextFormFields above it.
Since I only have 1 TextFormField outwidth the ExpansionPanelList, I've used _controllerDefaultDeposit.text to get the Deposit FormField value and manually validate it. It's a hacky solution, but it'll do for now.