The problem is with this code actually - var pageView = tab.get_pageView(); .
Here pageView is null since there is no MultiPageID associated with RadTabStrip. To avoid the problem, please try the following modification to your code:
RadTab tab =
RadPageView pageView =
RadTabStrip1.SelectedIndex = 0;
RadTabStrip1.MultiPageID = RadMultiPage1.ID;
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