QQuickFramebufferObject redraw

[themts]

Hey guys,

I created a QQuickFramebufferObject to render custom opengl content.
In QML I placed my component like this:
Renderer {
id: renderer
x: 72
y: 106
width: 496
height: 260
}
Renderer is a QQuickFramebufferObject::Renderer class.
I placed a button on my form to rerender like this:
button1.onClicked: renderer.update()

At first start of my application my render-function gets called and it draws as expected.
When I press the button, my render function get called again by the render-scheduler but this time I don't see anything.

I reduced my render function to some very simple gl-calls to be sure there is nothing else:
void NTestFboRenderer::render()
{
if (!first) {
glColor4f(1, 0, 0, 1);
glBegin(GL_LINES);
glVertex2d(0, 0);
glVertex2d(1, 1);
glEnd();
}
first = false;
}

Do you have any idea whats wrong here?
Do I have to mark my fbo as dirty or something?

EDIT:
glClear works all the time, but when I draw something like vertex I don't see it... (only on first render call)

[themts]

I solved this problem by creating a vertex- and fragment-shader because there was probably another shader active.

Is there a way to mark this topic as solved?