AEM Servlet Unit Test

I have the servlet below to use com.adobe.fd.forms.api.FormsService. I'm trying to create a unit test (see below) but I'm getting the following error. It's failing at line 33 of the servlet:

Document xmlDocument = new Document(xmlData.getBytes());

It seems that the servelt is not initialized correctly or messing the references. Can you please help?

Error:

Servlet:

import java.io.IOException;
import javax.servlet.Servlet;
import org.apache.commons.httpclient.HttpStatus;
import org.apache.commons.io.IOUtils;
import org.apache.sling.api.SlingHttpServletRequest;
import org.apache.sling.api.SlingHttpServletResponse;
import org.apache.sling.api.servlets.HttpConstants;
import org.civ.forms.core.services.document.FormsDocumentServices;
import org.osgi.framework.Constants;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.component.annotations.Reference;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import com.adobe.aemfd.docmanager.Document;
@Component(service = Servlet.class,
property = 
{ Constants.SERVICE_DESCRIPTION + "= ",
"sling.servlet.methods=" + HttpConstants.METHOD_POST, 
"sling.servlet.paths=/bin/render", 
"sling.auth.requirements=/bin/render"
})
public class FormRenderServlet extends org.apache.sling.api.servlets.SlingAllMethodsServlet {
@Reference
private transient FormsDocumentServices documentServices;
private static final String APPLICATION_PDF = "application/pdf";
private final transient Logger logger = LoggerFactory.getLogger(getClass());
@Override
protected void doPost(SlingHttpServletRequest request, SlingHttpServletResponse response) throws IOException {
try {
String xdpName = request.getParameter("xdpname");
String xmlData = request.getParameter("xmldata");
Document xmlDocument = new Document(xmlData.getBytes());
Document renderedPDF = documentServices.render(xdpName, xmlDocument);
String pdfFileName = xdpName.replace(".xdp", ".pdf");
response.setContentType(APPLICATION_PDF);
response.addHeader("Content-Disposition", "attachment; filename=" + pdfFileName);
response.setContentLength((int) renderedPDF.length());
IOUtils.copy(renderedPDF.getInputStream(), response.getOutputStream());
} catch (Exception e) {
response.setStatus(HttpStatus.SC_INTERNAL_SERVER_ERROR);
logger.error("reading or writing opertion failed : {}", e);
}
}
}

Test:

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;
import java.io.IOException;
import javax.servlet.ServletException;
import org.apache.sling.api.SlingHttpServletRequest;
import org.apache.sling.api.SlingHttpServletResponse;
import org.apache.sling.testing.mock.sling.junit.SlingContext;
import org.eclipse.jetty.http.HttpStatus;
import org.junit.Rule;
import org.junit.Test;
public class FormRenderServletTest {
    @Rule
    public final SlingContext context = new SlingContext();
    private FormRenderServlet fixture = new FormRenderServlet();
    @Test
    public void doGet() throws ServletException, IOException {
    SlingHttpServletRequest request = mock(SlingHttpServletRequest.class);       
        SlingHttpServletResponse response = mock(SlingHttpServletResponse.class);    
        
        when(request.getParameter("xdpname")).thenReturn("xdpTemplate.xdp");
        when(request.getParameter("xmldata")).thenReturn("");
        fixture.doPost(request, response);
        assertEquals(HttpStatus.OK_200, response.getStatus());
    }
}