Error:

The argument type 'void Function(String)' can't be assigned to the parameter type 'void Function(String?)?'.dartargument_type_not_assignable
abstract final class String implements Comparable<String>, Pattern
dart:core

Solution:

The error occurs because the onChanged callback in DropdownButtonFormField expects a function that takes a nullable string (String?), but your onChanged callback is specified to take a non-nullable string (String). To resolve this, you can update the type of newValue in your onChanged callback to be nullable:

DropdownButtonFormField<String>(
  isExpanded: true,
  icon: const Icon(Icons.keyboard_arrow_down),
  value: selectedDegree,
  hint: Text(
    'Type & Select Degree'), // Displayed when no option is selected
  items: degreeOptions.map((String degree) {
    return DropdownMenuItem<String>(
      value: degree,
      child: Text(degree),
    );
  }).toList(),
  onChanged: (String? newValue) {
    setState(() {
      selectedDegree =
          newValue ?? ''; // Update the selected degree when an option is chosen
    });
  },
  validator: (value) {
    if (value == null || value.isEmpty) {
      return 'Please select your Degree';
    }
    return null;
  },
),

In this updated code, newValue is now nullable (String?) to match the expected parameter type of onChanged. The ?? '' operator is used to provide a default value ('') in case newValue is null.