Why has to call glBindVertexArray(0); before call QPainter drawing operations in QOpenglWidget?

Question

Here is my program, I want to call OpenGL functions to draw a triangle,and then QPainter to draw a string. My widget code is :

class CMyTestOpenGLWidget : public QOpenGLWidget,protected QOpenGLFunctions_4_3_Core
{
public:
    CMyTestOpenGLWidget(QWidget* parent) : QOpenGLWidget(parent) {}

    void initializeGL() override
    {
        initializeOpenGLFunctions();
        ShaderInfo renderShaders[] = { {GL_VERTEX_SHADER,":shaders/shape.vert"},{GL_FRAGMENT_SHADER,":shaders/shape.frag"},{GL_NONE,NULL} };
        program = loadShaders(renderShaders);
        glGenVertexArrays(1, &vao);
        glBindVertexArray(vao);

        double triangle[] = {
            10,10,
            90,10,
            40,90
        };
        glGenBuffers(1,&vbo);
        glBindBuffer(GL_ARRAY_BUFFER,vbo);
        int nByte = sizeof(triangle);
        glBufferData(GL_ARRAY_BUFFER,nByte,triangle,GL_DYNAMIC_DRAW);

        glVertexAttribPointer(0, 2, GL_DOUBLE, GL_FALSE, 0, NULL);
        glEnableVertexAttribArray(0);

        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
    }

    void paintGL() override
    {
        mat4 model(1.0),view(1.0),pro(1.0);
        pro = glm::ortho(0.0f,100.0f,0.0f,100.0f,0.0f, 10.0f);
        pro = pro * view * model;
        glClear(GL_COLOR_BUFFER_BIT);
        glUseProgram(program);
        GLint loc = glGetUniformLocation(program,"pvm");
        glUniformMatrix4fv(loc, 1, GL_FALSE, glm::value_ptr(pro));
        glBindVertexArray(vao);
        glDrawArrays(GL_TRIANGLES,0,3);

        glBindVertexArray(0);
        QPainter p(this);
        p.setPen(Qt::red);
        p.drawText(QRect(0,0,50,100),"Test");
    }

    void resizeGL(int w, int h) override
    {
        glViewport(0, 0, w, h);
    }
private:
    uint loadShaders(ShaderInfo* shaders){...}
private:
    GLuint vao,
    vbo,
    program;
};

This program works, but I have to call glBindVertexArray(0); before drawing with QPainter. If I comment it out, there will be nothing but a black screen. Does the previous VAO bound to OpenGL affect latter drawing calls?