My function that lists the question and choices won't work. I keep getting the error code 'document.getElementById(...)' is null or not an object. The syntax appears to be correct.

I want the questions and choices to appear in the same divs. I do not want to list all my questions at once. When the user completes a question, they then move on to the next question which will appear in exactly the same divs as the first question until all the questions have been seen.

HTML Code:
<script>

var questions = new Array();
questions[0] = 'Is there a difference between a jungle and a rain forest?'
questions[1] = 'What is the world\'s most common religion?',
questions[2] = 'What is the second largest country (in size) in the world?';

var choices = new Array();
choices[0] = ['No difference', 'Some difference', 'Completely different'],
choices[1] = ['Christianity', 'Buddhism', 'Hinduism', 'Islam'],
choices[2] = ['USA', 'China', 'Canada', 'Russia'];

var answers = new Array();
answers[0] = ['Some difference'],
answers[1] = ['Christianity'],
answers[2] = ['Canada'];

var score = 0;
i= 0;

var listQuestion = function(){  
if(i<questions.length){
document.getElementById("myDiv1").innerHTML = '<p>'+questions[i]+'</p>';
for (k=0; k<choices[i].length; k++){
document.getElementById("myDiv2").innerHTML ='<p><input type = "radio" name = "questionchoice">'+choices[i][k]+'</p>';
}
document.getElementById("myDiv3").innerHTML = '<p><button onClick = "getRadioValue()">Check</button></p> <br>';
};
};



var getRadioValue = function()
{
    for (var h = 0; h < document.getElementsByName('questionchoice').length; h++)
    {
        var value = '';

        if (document.getElementsByName('questionchoice')[h].checked==true)
        {
            value = document.getElementsByName('questionchoice')[h].value;


    score+=1        

}
    }
if (value== answers[i]){
    document.getElementById("myDiv4").innerHTML ="That is correct. </br><button input type = 'submit' onClick = 'loadContent()'> Next Question</button>";   
}

else {

document.getElementById("myDiv4").innerHTML ="That is incorrect. </br><button input type = 'submit' onClick = 'loadContent()'> Next Question</button>"; 

    }
i++;
};


var whatIsScore = function(){

return score; 

}
window.onload = listQuestion();

</script>

</head>

<body>
<div id="myDiv1"></div> 
<div id="myDiv2"></div>
<div id="myDiv3"></div>
<div id="myDiv4"></div>


</body>