Getting an error when trying this challenge. Please see below error. 

Challenge Not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.NullPointerException: Attempt to de-reference a null object


To pass this challenge, create an Apex class that inserts a new account named after an incoming parameter. If the account is successfully inserted, the method should return the account record. If a DML exception occurs, the method should return null.
The Apex class must be called 'AccountHandler' and be in the public scope.
The Apex class must have a public static method called 'insertNewAccount'.
The 'insertNewAccount' method must accept an incoming string as a parameter, name the account after the parameter, insert it into the system and then return the account record.
The 'insertNewAccount' method must also accept an empty string, catch the failed DML and return null.

Hi 
I am using my salesforce userid and password to login to Developer edition and it is not working and showing me error. 
Please check your username and password. If you still can't log in, contact your Salesforce administrator. 
Please help 
user id pbiswal@salesforce.com

Create an Apex class that returns an array (or list) of strings: 
Create an Apex class that returns an array (or list) of formatted strings ('Test 0', 'Test 1', ...). The length of the array is determined by an integer parameter.The Apex class must be called 'StringArrayTest' and be in the public scope.
The Apex class must have a public static method called 'generateStringArray'.
The 'generateStringArray' method must return an array (or list) of strings. Each string must have a value in the format 'Test n' where n is the index of the current string in the array. The number of returned strings is specified by the integer parameter to the 'generateStringArray' method.

MyApexClass to above Challenge:

public class StringArrayTest {
    
    public static List<string> generateStringArray(Integer n)
    {
        List<String> myArray = new List<String>();
        
        for(Integer i=0;i<n;i++)
        {
            myArray.add('Test'+i);
            System.debug(myArray[i]);
        }
        return myArray;
        
    }

It's compile and execute as per requirement in Developer console. But Traihead showing the below error:

Challenge not yet complete... here's what's wrong: 
Executing the 'generateStringArray' method failed. Either the method does not exist, is not static, or does not return the proper number of strings.

Anyhelp would be greatly appreciated. Thanks.