Android : Getting OutOfBoundsException while pagination a String

I am creating an epub reader for android. For the pagination part, I am trying to get the whole string content and then search for space in the string. Then I get the text height and compare it with the screen height. if still (text height < screen height) I loop through the string and do the same thing in a while loop.

Every thing went well, but when it comes to the end of the string I get IndexOutOfBoundsException. I have attached the screenshot of the Logcat below.

The code I used to get the no of pages is like this

public String getNoOfPages(String text){


    String remainingString = "";

    try{
        int screenHeight = getScreenHeight();
        String originalText = text;
        String strToModify = text;


        StringBuilder newString = new StringBuilder();
        StringBuilder oldString = new StringBuilder();


        int startIndex = 0;
        String strToFind = " ";
        int index = strToModify.indexOf(strToFind,startIndex);

        newString.append(originalText.substring(startIndex, index+1));
        oldString.append(newString.toString());
        startIndex = index+1;

        int textHeight = getTextHeight(newString.toString());

        while(textHeight < screenHeight){
            index = strToModify.indexOf(strToFind,startIndex);
            oldString.replace(0,oldString.toString().length(),newString.toString());
            newString.append(originalText.substring(startIndex, index+1));
            startIndex = index+1;
            textHeight = getTextHeight(newString.toString());
        }

        remainingString = originalText.substring(oldString.length()-1,originalText.length());

    }catch(Exception e){
        Log.d("chathura123","Error in getNoOfPages  " );
        e.printStackTrace();

    }

    return remainingString; 
}

The logic is when the remaining string is an empty string("") ,it means that is the end of the content of the page. So I want to check until it returns an empty string.

What is the wrong with this? Why I am getting OutOfBoundsException?

Thank you.