makeCurrent() needed inside of resizeGL()

#include <QApplication>
#include <QMainWindow>
#include <QOpenGLFunctions_2_1>
#include <QOpenGLWidget>

class GL21Widget : public QOpenGLWidget, public QOpenGLFunctions_2_1 {
  void initializeGL() override {
    initializeOpenGLFunctions();
  }
  void resizeGL(int width, int height) override {
    makeCurrent();
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0.0, 1.0, 0.0, 1.0, 0.0, 1.0);
  }
  void paintGL() override {
    glClear(GL_COLOR_BUFFER_BIT);
    glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
    glBegin(GL_TRIANGLES);
    glColor3d(1.0, 0.0, 0.0);
    glVertex3d(0.5, 1.0, 0.0);
    glColor3d(0.0, 1.0, 0.0);
    glVertex3d(0.0, 0.0, 0.0);
    glColor3d(0.0, 0.0, 1.0);
    glVertex3d(1.0, 0.0, 0.0);
    glEnd();
  }
};
class GL21Window : public QMainWindow {
public:
  GL21Window() {
    setCentralWidget(new GL21Widget{});
    setGeometry(500, 200, 750, 750);
    show();
  }
};
int main(int argc, char* argv[]) {
  auto app = QApplication{argc, argv};
  auto win = GL21Window{};
  return QApplication::exec();
}