title: Setting AssociatedControlID on label fails
body: <p>I have a composite control that adds a TextBox and a Label control to its Controls collection. When i try to set the Label's AssociatedControlID to the ClientID of the Textbox i get this error</p>
  
  <pre><code>Unable to find control with id 
  'ctl00_MainContentPlaceholder_MatrixSetControl_mec50_tb'
  that is associated with the Label 'lb'. 
  </code></pre>
  
  <p>Ok so a little background. I got this main-composite control that dynamically adds a number of 'elements' to its control collection. One of these elements happen to be this 'MatrixTextBox' which is the control consisting of a TextBox and a Label.</p>
  
  <p>I hold the Label and TextBox as protected class variables and init them in CreateChildControls:</p>
  
  <pre><code>    ElementTextBox = new TextBox();
      ElementTextBox.ID = "tb";
      Controls.Add(ElementTextBox);
  
      ElementLabel = new Label();
      ElementLabel.ID = "lb";
      Controls.Add(ElementLabel);
  </code></pre>
  
  <p>I tried setting the</p>
  
  <pre><code>ElementLabel.AssociatedControlID = ElementTextBox.ClientID;
  </code></pre>
  
  <p>both right after adding the controls to the Controls collection and even in PreRender - both yield the same error. What am i doing wrong?</p>