NullPointerException in parsing single quotes

[oncebasun]

In QuotationAnnotator.java

if (usesSingleQuotes) {
			
    Token previousToken = null;
    
    // fixes a problem with single quotes before "I" not being recognized as open quotes
    for (int i = 0; i < book.tokens.size(); i++) {
        Token token = book.tokens.get(i);
        if (token.original.equals("'")) {
            if (!previousToken.whitespaceAfter.equals("")) {
                token.lemma="`";
                token.word="`";
                token.pos="``";
            }
        }
        previousToken=token;
    }
    
    
    previousToken = null;

    for (int i = 0; i < book.tokens.size(); i++) {
        Token token = book.tokens.get(i);
        if (token.lemma.equals("`")) {
            
            if (!previousToken.whitespaceAfter.equals("")) {
                open = true;
                start = token.tokenId;
            }
        } else if (token.lemma.equals("'")) {
            end = token.tokenId;

            if (start > -1 && open) {
                Quotation quote = new Quotation(start, end,
                        token.sentenceID);
                quotations.put(start, quote);
            }
            start = -1;

            open = false;

        }
        previousToken = token;

    }
}

will cause NullPointerException when previousToken is null.