Nullpointer Exception when converting DOCX to PDF using Docx4j with non-XSL transformation

Question

I am fairly new to Docx4j library. I'm trying to convert a DOCX file to PDF using the Docx4j library and I want to use the non-XSL transformation method (Docx4J.FLAG_EXPORT_PREFER_NONXSL) to reduce the overall processing/conversion time (the whole conversion process is taking approximately 2mins). But I'm encountering an exception related to image handling.

Here's the exception I'm facing:

java.lang.NullPointerException: Cannot invoke "org.docx4j.model.images.WordXmlPictureE20.createXslFoImageElement()" because "converter" is null
at org.docx4j.model.images.WordXmlPictureE20.createXslFoImgE20 (WordXmlPictureE20.java)

The above exception suggests that there's an issue with image conversion during the DOCX to PDF transformation. However, I'm not sure why this is occurring as I expect Docx4j to handle images with its default settings.

Here is the code snippet that leads to the exception:

import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;
import org.docx4j.Docx4J;
import org.docx4j.convert.out.FOSettings;
import org.docx4j.openpackaging.packages.WordprocessingMLPackage;

public class DocxToPDFConverter {

    public static void convertToPDF(File docxFile, File pdfFile) throws Exception {
        // Load the DOCX into a WordprocessingMLPackage
        WordprocessingMLPackage wordMLPackage = Docx4J.load(docxFile);
        wordMLPackage.setFontMapper(new BestMatchingMapper());

        // Configure FO settings
        FOSettings foSettings = Docx4J.createFOSettings();
        foSettings.setWmlPackage(wordMLPackage);
        foSettings.setApacheFopMime(FOSettings.MIME_PDF);

        // Prepare the output stream
        try (OutputStream out = new FileOutputStream(pdfFile)) {
            // Convert to PDF with non-XSL transformation
            Docx4J.toFO(foSettings, out, Docx4J.FLAG_EXPORT_PREFER_NONXSL);
        }
    }

Attempts to Resolve:

Ensured all required jars are on the classpath. I have docx4j-JAXB-ReferenceImpl, docx4j-export-fo of version 11.4.9 in my pom.xml

Experimented with FLAG_EXPORT_PREFER_NONXSL for a faster conversion, but no significant improvement in response time was noted because of exception.

Checked the image handler setup as per default Docx4J configuration.

Questions:

1. How can I fix the "converter is null" issue to successfully complete the DOCX to PDF conversion?

2. Are there any optimizations I can apply to reduce the response time when converting documents using Docx4J.toPDF()? I have already tried using the non-XSLT based conversion (FLAG_EXPORT_PREFER_NONXSL), but the performance improvement isn't sufficient.

Environment:

Java version: 17, and Docx4J version: 11.4.9 (docx4j-export-fo in classpath)

reference: https://github.com/plutext/docx4j/blob/VERSION_11_4_8/docx4j-samples-docx-export-fo/src/main/java/org/docx4j/samples/ConvertOutPDFviaXSLFO.java

Any insights, suggestions, or optimizations that could address these issues would be highly appreciated. Thank you for your time and help!