The “type ‘Null’ is not a subtype of type ‘String’” error typically occurs when trying to access a property of an object that is null. In Flutter, this often happens when using the null-aware operator (??) or when accessing properties of widget parameters that could be null.

Causes of Null Values: Null values can arise from various sources in a Flutter application:

1. Widget Parameters: When passing data to a widget using constructor parameters, the data might be null if not provided or initialized properly.
2. Asynchronous Operations: Data fetched asynchronously from APIs or databases might not be available immediately, leading to null values until the operation completes.
3. Optional Data: Some properties of objects may be optional and could be null under certain conditions.

Solutions:

1. Null-aware Operators:
   • Use the null-aware operator (??) to provide default values for nullable variables.
   • Example: String name = nullableName ?? 'Unknown';
2. Null Checking:
   • Check if a variable is null before accessing its properties.
   • Example:

if (data != null) {
  // Access properties of data
} else {
  // Handle the case where data is null
}

Default Values:

• Provide default values when accessing properties that might be null.

Example

String regNumber = widget.registrationData['regnumber'] ?? '';

Asynchronous Operations:

• Use FutureBuilder to handle asynchronous operations and display loading indicators or error messages while waiting for data.
• Check for null data before accessing it in the builder function.

In my case:

@override
  void initState() {
    super.initState();
    fetchCouncils(); // Fetch councils when the widget initializes
    regNumberController.text = widget.registrationData['regnumber'] ?? '';
    _dropdownValue = widget.registrationData['regcouncil'] ?? 'Select Council'; // Initialize as null
    _selectedYear = widget.registrationData['regyear'] ?? null;
    }

After changing

@override
void initState() {
    super.initState();
    fetchCouncils(); // Fetch councils when the widget initializes
    if (widget.registrationData != null) {
      regNumberController.text = widget.registrationData['regnumber'] ?? '';
      _dropdownValue =
          widget.registrationData['regcouncil'] ?? 'Select Council';
      _selectedYear = widget.registrationData['regyear'] ??
          yearsList.first; // Use a default value if regyear is null
    } else {
      // Handle the case where widget.registrationData is null
    }
  }