Stack trace of the crash?
Nope, changed to a regular sf::RenderTexture created at each rendering cycle, no change. Now, there's no crash, but the SFML part just doesn't show.
Your System::Render(), shouldn't you only be conditionally calling renderWidgets() if the container was updated? Something like:
void System::Render()
{
windowMain.clear(sf::Color(50,0,0));
renderOpenGL();
if (primaryContainer.wasUpdated())
{
windowMain.pushGLStates();
renderWidgets();
windowMain.popGLStates();
}
windowMain.display();
numberOfFrames++;
}