How to fix 'Unhandled exception at <memory location> in <program name>. Access violation writing location <memory location>.' in visual studio 2017

I am writing code to make an invoice in excel by using the library libxl. My code creates a file, writes in it and then saves the file in the project folder.

However, as I progress to debug it, I see an 'Unhandled exception', with access violation in writing a particular location. The exception actually says this 'Unhandled exception at 0x100437D7(libxl.dll) in BillingSoft.exe:0xC0000005: Access violation writing location 0x00C3DED4.'. I have read many articles and solutions but none of them seem to fix my problem. I tried allocating space for "book" by using

    Book* book = (Book*)malloc(sizeof(book));

before declaration of "book", but it just changes the location values and doesn't solve the problem.

This is my main function:

    int main()
    { 
    Book* book = xlCreateBook(); // xlCreateXMLBook() for xlsx
if (book)
{
    Font* boldFont = book->addFont();
    boldFont->setBold();

    Font* titleFont = book->addFont();
    titleFont->setName(L"Arial Black");
    titleFont->setSize(16);

    Format* titleFormat = book->addFormat();
    if (titleFormat = nullptr)
    {
        titleFormat->setFont(titleFont);
    }

    Format* headerFormat = book->addFormat();
    if (headerFormat = nullptr)
    {
        headerFormat->setAlignH(ALIGNH_CENTER);
        headerFormat->setBorder(BORDERSTYLE_THIN);
        headerFormat->setFont(boldFont);
        headerFormat->setFillPattern(FILLPATTERN_SOLID);
        headerFormat->setPatternForegroundColor(COLOR_TAN);
    }

    Format* descriptionFormat = book->addFormat();
    if (descriptionFormat = nullptr)
    {
        descriptionFormat->setBorderLeft(BORDERSTYLE_THIN);
    }

    Format* amountFormat = book->addFormat();
    if (amountFormat = nullptr)
    {
        amountFormat->setNumFormat(NUMFORMAT_CURRENCY_NEGBRA);
        amountFormat->setBorderLeft(BORDERSTYLE_THIN);
        amountFormat->setBorderRight(BORDERSTYLE_THIN);
    }

    Format* totalLabelFormat = book->addFormat();
    if (totalLabelFormat = nullptr)
    {
        totalLabelFormat->setBorderTop(BORDERSTYLE_THIN);
        totalLabelFormat->setAlignH(ALIGNH_RIGHT);
        totalLabelFormat->setFont(boldFont);
    }
    Format* totalFormat = book->addFormat();
    if (totalFormat = nullptr)
    {
        totalFormat->setNumFormat(NUMFORMAT_CURRENCY_NEGBRA);
        totalFormat->setBorder(BORDERSTYLE_THIN);
        totalFormat->setFont(boldFont);
        totalFormat->setFillPattern(FILLPATTERN_SOLID);
        totalFormat->setPatternForegroundColor(COLOR_YELLOW);
    }
    Format* signatureFormat = book->addFormat();
    if (signatureFormat = nullptr)
    {
        signatureFormat->setAlignH(ALIGNH_CENTER);
        signatureFormat->setBorderTop(BORDERSTYLE_THIN);
    }

    Sheet* sheet = book->addSheet(L"Sheet1");
    if (sheet)
    {
        sheet->writeStr(2, 1, L"Invoice No. 3568", titleFormat);

        sheet->writeStr(4, 1, L"Name: John Smith");
        sheet->writeStr(5, 1, L"Address: San Ramon, CA 94583 USA");

        sheet->writeStr(7, 1, L"Description", headerFormat);
        sheet->writeStr(7, 2, L"Amount", headerFormat);

        sheet->writeStr(8, 1, L"Ball-Point Pens", descriptionFormat);
        sheet->writeNum(8, 2, 85, amountFormat);
        sheet->writeStr(9, 1, L"T-Shirts", descriptionFormat);
        sheet->writeNum(9, 2, 150, amountFormat);
        sheet->writeStr(10, 1, L"Tea cups", descriptionFormat);
        sheet->writeNum(10, 2, 45, amountFormat);

        sheet->writeStr(11, 1, L"Total:", totalLabelFormat);
        sheet->writeNum(11, 2, 280, totalFormat);

        sheet->writeStr(14, 2, L"Signature", signatureFormat);

        sheet->setCol(1, 1, 40);
        sheet->setCol(2, 2, 15);

    }
    book->save(L"Invoice.xls"); // exception here!
    book->release();
}
return 0;
    }

I just want the program to work fine without any error. Please help me fix my problem. Any help would be appreciated !

You are always overwriting everything Excel gives you. So when you do:

sheet->writeStr(2, 1, L"Invoice No. 3568", titleFormat);

You pass in titleFormat which is a nullptr because of line:

if (titleFormat = nullptr)

Turn on your warnings and fix your code:

if (titleFormat != nullptr)

And same for all other if.