Errors connected with creating QMathGL class in application

Question

I have a problem with MathGL library and creating MathGL instance in the application. Every time I try to run it there is an error saying that QApplication must be constructed before QWidget (from which the class QMathGL inherits). Below you can find my code of main and all the functions connected with the MainWindow class:

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "functions.h"
#include "surface.h"
#include "error.h"
#include <QString>
#include <QFileDialog>
#include <GL/glut.h>
#include <cmath>
#include <fstream>
#include <mgl2/qmathgl.h>

surface graph;

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_draw_clicked()
{
    string s;
    bool correct = 1;
    error *errorWindow = new error(this);
    s = ui->eqEdit->text().toStdString();
    period fPeriod = period(ui->upEdit->text().toDouble(),ui->lowEdit->text().toDouble());
    functionSum fSum = functionSum(s,fPeriod,ui->accEdit->text().toDouble());
    for (int i = 0; i < fSum.accuracy; i++)
    {
        for (int j = 0; j < fSum.accuracy; j++)
        {
            if(isnan(fSum.zValues[i][j]) || isinf(fSum.zValues[i][j]))
            {
                correct = 0;
            }
        }
    }
    if(!correct)
    {
        errorWindow->show();
    }
    graph = surface(fSum);
}


void MainWindow::on_save_clicked()
{
    QString plik;
    string splik;
    plik = QFileDialog::getSaveFileName(this,"Zapisz plik",QString(),"Pliki tekstowe (*.txt)");
    splik = plik.toStdString();
    ofstream saver;
    saver.open(splik.c_str());
    if (saver.is_open())
    {
        for(int i = 0; i < graph.points.size(); i++)
        {
            saver << graph.points[i].x << " " << graph.points[i].y << " " << graph.points[i].z << endl;
        }
        saver.close();
    }
}

void MainWindow::on_load_clicked()
{
    QString plik;
    string splik;
    plik = QFileDialog::getOpenFileName(this,"Otwórz plik",QString(),"Pliki tekstowe (*.txt)");
    splik = plik.toStdString();
    ifstream loader;
    loader.open(splik.c_str());
    graph.points.clear();
    if(loader.is_open())
    {
        while(!loader.eof())
        {
            point a(0,0,0);
            loader >> a.x >> a.y >> a.z;
            graph.points.push_back(a);
        }
        loader.close();
    }
}

void MainWindow::on_plot_clicked()
{
     int a = sqrt(graph.points.size());
     mglData x(a,a);
     mglData y(a,a);
     mglData z(a,a);
     long long int k = 0;
     for (long long int i = 0;i<sqrt(graph.points.size());i++)
     {
         for (long long int j = 0;j<sqrt(graph.points.size());j++)
         {
             x.a[i+a*j] = graph.points[k].x;
             y.a[i+a*j] = graph.points[k].y;
             z.a[i+a*j] = graph.points[k].z;
             k++;
         }
     }
     mglGraph plot;
     plot.Rotate(50,60);
     plot.Light(true);
     plot.Surf(x,y,z);
     QMathGL plotter;
     plotter.setGraph(&plot);
     plotter.show();
}

main.cpp

#include "mainwindow.h"
#include <QApplication>


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}