Error #2007: Parameter text must be non-null

in my program, the user has to input a number between 10 and 99 and the program will convert the number into words. The program somewhat works; however, when I input a number between 20 and 99, say for example, 45, the program will print out, "Forty fiveForty." This is when "Error #2007: Parameter text must be non-null," appears in the output section. I also can't seem to make numbers from 11 to 19 to work. Instead of showing the number in words, the result says "Error." Kindly edit my code in order for the program to function correctly and print out a number between 10 and 99 correctly.

// This line makes the button, btnConvert wait for a mouse click
// When the button is clicked, the convertNumber function is called
btnConvert.addEventListener(MouseEvent.CLICK, convertNumber);

// These lines make the textinputs wait for a mouse click
// When any of these components are clicked, the clearLabels function is called
txtinNumber.addEventListener(MouseEvent.CLICK, clearLabels);

// Declare Global Variables
var num:int;         // number from 10 - 99
var tensDigit:int;   // the tens digit
var onesDigit:int;   // the ones digit
var teensDigit = [11, 12, 13, 14, 15, 16, 17, 18, 19];

// This is the convertNumber function
// e:MouseEvent is the click event experienced by the button
// void indicates that the function does not return a value
function convertNumber(e:MouseEvent):void
{ 
    getData();
    if (num < 10 || num > 99){
        lblOutput.text = "Invalid number. Enter a number between 10 and 99 inclusive.";
    }
    else{
        lblOutput.text = "";
        if (num >= 20) {
            tensDigit = Math.floor(num / 10);
            onesDigit = num % 10;
            tens();
            ones();
        }
        else{
            tensDigit = Math.floor(num / 10);
            onesDigit = num % 10;
            teens();
        }
    }
    lblOutput.text = 
tens();
lblOutput.text += onesDigit
ones();




}

// This is the getData function
// It gets the number from the user
function getData()
{
    // complete the code here
    num = int(txtinNumber.text);
}

// This is the tens function
// It outputs the word representation of 20, 30, 40,..,90
function tens()
{
    if (tensDigit == 2 && tensDigit < 3)
    {
    lblOutput.text += "Twenty";
    }

    else if (tensDigit == 3 && tensDigit < 4)
    {
    lblOutput.text += "Thirty";
    }

    else if (tensDigit == 4 && tensDigit < 5)
    {
    lblOutput.text += "Forty";
    }

    else if (tensDigit == 5 && tensDigit < 6)
    {
    lblOutput.text += "Fifty";
    }

    else if (tensDigit == 6 && tensDigit < 7)
    {
    lblOutput.text += "Sixty";
    }

    else if (tensDigit == 7 && tensDigit < 8)
    {
    lblOutput.text += "Seventy";
    }

    else if (tensDigit == 8 && tensDigit < 9)
    {
    lblOutput.text += "Eighty";
    }

    else if (tensDigit == 9 && tensDigit < 10)
    {
    lblOutput.text += "Ninety";
    }

    else
    {
    lblOutput.text += "Unknown."
    }

}

// This is the ones function
// It outputs the word representaion for any number from 1 - 9 inclusive
function ones()
{
    if (onesDigit == 1)
    {
        lblOutput.text += " one"
    }

    else if (onesDigit == 2)
    {
        lblOutput.text += " two"
    }

    else if (onesDigit == 3)
    {
        lblOutput.text += " three"
    }

    else if (onesDigit == 4)
    {
        lblOutput.text += " four"
    }

    else if (onesDigit == 5)
    {
        lblOutput.text += " five"
    }

    else if (onesDigit == 6)
    {
        lblOutput.text += " six"
    }

    else if (onesDigit == 7)
    {
        lblOutput.text += " seven"
    }

     else if (onesDigit == 8)
    {
        lblOutput.text += " eight"
    }

    else if (onesDigit == 9)
    {
        lblOutput.text += " nine"
    }



}

// This is the teens function
// It outputs the word representation for any number from 10 - 19 inclusive
function teens()
{
    if (teensDigit == 10)
    {
        lblOutput.text += "Ten"
    }

    else if (teensDigit == 11)
    {
        lblOutput.text += "Eleven"
    }

    else if (teensDigit == 12)
    {
        lblOutput.text += "Twelve"
    }

    else if (teensDigit == 13)
    {
        lblOutput.text += "Thirteen"
    }

    else if (teensDigit == 14)
    {
        lblOutput.text += "Fourteen"
    }

    else if (teensDigit == 15)
    {
        lblOutput.text += "Fifteen"
    }

    else if (teensDigit == 16)
    {
        lblOutput.text += "Sixteen"
    }

    else if (teensDigit == 17)
    {
        lblOutput.text += "Seventeen"
    }

    else if (teensDigit == 18)
    {
        lblOutput.text += "Eighteen"
    }

    else if (teensDigit == 19)
    {
        lblOutput.text += "Nineteen"
    }

    else
    {
        lblOutput.text = "Error."
    }



}

// This is the clearLabels function
// e:MouseEvent is the click event experienced by the textInput
// void indicates that the function does not return a value
function clearLabels(e:MouseEvent):void
{
     lblOutput.text = "";
}

You're referencing the teensDigit as if it were an int, but you created an array. Change your teens() function to

if (num == 10) {

and so on, rather than teensDigit ==

Alternatively, you could simply use a lookup table. Below is a simplified version of what you were trying to do which can be compiled on a clean project.

convertNumber(7)    // Outputs: seven
convertNumber(13)   // Outputs: Thirteen
convertNumber(56)   // Outputs: Fifty six

function convertNumber(i:int):void { 
    var tens:int = Math.floor(i / 10);
    var ones:int = (i > 9 && i < 20) ? i : i % 10;

    trace(lookup.double[tens] + lookup.single[ones])
}

var lookup:Object = {
    "single":[
        "",
        "one",
        "two",
        "three",
        "four",
        "five",
        "six",
        "seven",
        "eight",
        "nine",
        "Ten",
        "Eleven",
        "Twelve",
        "Thirteen",
        "Fourteen",
        "Fifteen",
        "Sixteen",
        "Seventeen",
        "Eighteen",
        "Nineteen"
    ],
    "double":[
        "",
        "",
        "Twenty ",
        "Thirty ",
        "Forty ",
        "Fifty ",
        "Sixty ",
        "Seventy ",
        "Eighty ",
        "Ninety ",
    ]
}