Internal Server Error when calling service on retrofit2

Question

I am using Retrofit2 and the problem is that in my onResponse I get internal server error.

I have made a Call class i.e

public class QuestionCreate {

private String status;
private String result;
private String error;

public String getStatus() {
    return status;
}

public void setStatus(String status) {
    this.status = status;
}

public String getResult() {
    return result;
}

public void setResult(String result) {
    this.result = result;
}

public String getError() {
    return error;
}

public void setError(String error) {
    this.error = error;
}
}

My interface is :

public interface MultipartApi {

@Multipart
@POST("webapi/groups/questions/create")
Call<QuestionCreate> createQuestion(@Part("questionText") String questionText, @Part("image") byte[] image, @Part("createdBy") String createdBy, @Part("visibleToMembers") String visibleToMembers, @Part("groupId") String groupId, @Part("questionOptions") String questionOptions, @Part("questionTypeId") String questionTypeId, @Part("isAnswerEditable") String isAnswerEditable);

}

Well just for more detail , this is my small server method of createQuestion:

@POST
@Path("/create")
@Produces(MediaType.APPLICATION_JSON)
@Consumes(MediaType.MULTIPART_FORM_DATA)
public JSONObject createQuestion(@FormDataParam("questionText") String questionText, @FormDataParam("image") InputStream image, @FormDataParam("createdBy") String createdBy, @FormDataParam("visibleToMembers") String visibleToMembers, @FormDataParam("groupId") String groupId, @FormDataParam("questionOptions") String questionOptions, @FormDataParam("questionTypeId") String questionTypeId, @FormDataParam("isAnswerEditable") String isAnswerEditable, @FormDataParam("imageFormat") String imageFormat) {
    String message = "";    
    JSONObject obj = new JSONObject();
    Question question = new Question();
    System.out.println("Inside createQuestion of class "+this.toString());      
    try {
            question.setQuestionText(questionText);             question.setCreatedBy(createdBy);                               
            question.setVisibleToMembers(visibleToMembers);
            question.setGroupId(groupId);
            question.setQuestionOptions(questionOptions);
            question.setQuestionTypeId(questionTypeId); 
            question.setIsAnswerEditable(isAnswerEditable);             
            DBConnection.insertQuestion(question, image, servletContext, imageFormat);
            message = "Question Created";
            obj.put("status", 1);
            obj.put("result", message);
            return obj;
        } catch(SQLException sqle){
            System.out.println("createQuestion catch sqle");
            message = "A problem occured while creating question : "+sqle.getMessage();
            try {
                obj.put("status", 501);
                obj.put("error", message);
            } catch (JSONException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }               
            return obj;
        }
        catch (Exception e) {
            // TODO Auto-generated catch block
            System.out.println("Inside createQuestion catch  "+e.getMessage());
            message = "A problem occured while creating question : "+e.getMessage();
            try {
                obj.put("status", 501);
                obj.put("error", message);
            } catch (JSONException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }               
            return obj;
        }       
}

and i am calling my service like:

                Retrofit retrofit = new Retrofit.Builder()
                    .baseUrl(URLConstants.URL_BASE+"/")
                    .addConverterFactory(GsonConverterFactory.create())
                    .build();

            MultipartApi multipartApi = retrofit.create(MultipartApi.class);

            Call<QuestionCreate> req = multipartApi.createQuestion(question.getQuestionText(), inputStream, question.getCreatedBy(), question.getVisibleToMembers(), question.getGroupId(), question.getQuestionOptions(), question.getQuestionTypeId(), question.getIsAnswerEditable());
            req.enqueue(new Callback<QuestionCreate>() {
                @Override
                public void onResponse(Call<QuestionCreate> call, Response<QuestionCreate> response) {
                    String str = response.message();
                }

                @Override
                public void onFailure(Call<QuestionCreate> call, Throwable t) {
                     //Print Error
                }
            });

I have just started learning about it yesterday so there must be some mistakes. So I have a few questions:

1. How do I just send a map of data (like in StringRequest of volley)?
2. How do I get string as a response because I didn't want a model class for this purpose i.e how do I get the just json string?
3. What could be the reason for getting internal server error?

Thank you.