Qt glTexSubImage2d not displaying texture

Question

I'm trying to display a texture within a QOpenGlWidget in a QMainWindow. For that I made a class which derived from QopenGlWidget and my corresponding widget is promoted to use that class.

class GLWidget : public QOpenGLWidget, protected QOpenGLFunctions
{
    Q_OBJECT
...
}

My plan is to first initlialize the texture once in the initializeGL() method, then use the texture id in the paintGL() method and only update it with glTexSubImage2D().

GLWidget::initializeGL()
{
...
  glTexImage2D(...);
...
}
GLWidget::paintGL()
{
...
   glTexSubImage2D(...);
...
}

This works ONLY if I use glTexImage() right before glTexSubImage2D() in the paintGL() method. Otherwise the texture won't be drawn. I think there is a problem with the drawing context. But I cannot figure it out.

I set the new texture from the outside with a the method void setTexture(...); This method also calls update() to repaint the widget. Is it possible that calling this method from an outside thread causes the texture to render in another context?

I tried to set a shared context in the paintGl() method. I used the context which was used in the initializeGL() method. Also, I tried do make the context from initializeGL() the current context while drawing in painGL(). Again same result. Here are my OpenGL functions for creating the texture:

//Works when used in paintGl().
glTexImage2D(GL_TEXTURE_2D,0, GL_RGB, m_cvImage.cols, m_cvImage.rows,0, GL_BGR, GL_UNSIGNED_BYTE, m_cvImage.data);

//Not working when used in paintGl() if glTexImage2D is used to create the texture in initializeGl().
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, m_cvImage.cols, m_cvImage.rows, GL_BGR,, GL_UNSIGNED_BYTE, m_cvImage.data);

//Also working in paintGL()
glTexImage2D(GL_TEXTURE_2D,0, GL_RGB, m_cvImage.cols, m_cvImage.rows,0, GL_BGR, GL_UNSIGNED_BYTE, NULL);
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, m_cvImage.cols, m_cvImage.rows, GL_BGR, GL_UNSIGNED_BYTE, m_cvImage.data);

When I try to call makeCurrent() in the paintGL() method (even if it should be called automatically right before) I get an Assertion:

ASSERT: "context" in file opengl\qopenglfunctions.cpp, line 209

The same applies to the doneCurrent() method (just for testing). This tells me that there is no available context for the paintGL() method. Yet a check with this seems to be fine.

if (context() == 0)...