OpenGL/FBO/QML - How to correctly render using FBO?

[0x5FB5]

I tried to make a test OpenGL project using FBO. I've made it like in Qt's example. To test, I call glClear function, which fills the window to green color, after which I draw a simple big red point, but every frame I see only a blank screen. I can see the point for a few seconds when I resize the screen, also point resets the size, color and rendering coordinates (to left bottom angle of the window).

I'm really new in OpenGL and maybe it's my problem in rendering part, but I don't understand what's wrong.
Source code link

[0x5FB5]

I've found another solution for me, that uses QuickItem class without any problems with rendering
GitHub link