The non-nullable variable '_dio' must be initialized.

This error message is indicating that there is a variable named '_dio' which has been declared as non-nullable, but it has not been initialized with a value. Non-nullable variables are variables that must always have a value assigned to them, and cannot be null.

To fix this error, you need to initialize the '_dio' variable with a value before it is used. Depending on the type of the variable, you may need to assign it a default value or initialize it with a constructor or a method call. For example:

import 'package:dio/dio.dart';

void main() {
  Dio _dio = Dio(); // initialize the _dio variable with a new instance of Dio
  // use the _dio variable here
}

In this example, the '_dio' variable is initialized with a new instance of the 'Dio' class, which is imported from the 'dio' package. You can replace this with the appropriate initialization for your specific use case.