Generally placing the RadWindow in an update panel is not recommended, because the rendered HTML is moved
from its original location in the markup. This will most often result in
the expected functionality simply not working. Please take a look into this for more information.
Setting VisibleOnPageLoad property of RadWindow to false doesn't close the RadWindow. Please try the following code snippet to close the RadWindow.
Didn't work if I click the button for multiple times I am getting the following error
Invalid postback or callback argument. Event validation is
enabled using <pages enableEventValidation="true"/> in
configuration or <%@ Page EnableEventValidation="true" %> in a
page. For security purposes, this feature verifies that arguments to
postback or callback events originate from the server control that
originally rendered them. If the data is valid and expected, use the
ClientScriptManager.RegisterForEventValidation method in order to
register the postback or callback data for validation.