The issue you describe might be related either to defining a proper timeout before getting the object reference inside the RadWindow or to the code you’re using to get the object reference.
Could you please provide the following additional information in order to get a better understanding on your setup?
1. How is your RadWindow configured – do you use a ContentTemplate or you load an external page through its url? This information is needed because in those two scenarios referencing the controls inside the RadWindow is done in a different manner.
2. Could you please share the code snippet which shows how you get the object reference ?
Once we receive this, we will do our best to help.
the Telerik team