Note that, RadTabControl loads the content of its tabs on demand. In other words it loads the Content of a certain tab when the tab is selected and then when the selection is changed the Content of the new selected tab is loaded and the old content is removed from the visual tree. So, if the RadHtmlPlaceholder control is placed into a tab that is not selected, you won't be able to get its HTML presenter.
You can alter this behavior by setting the tab control's IsContentPresevered property to True. This will prevent the control from destroying the Content of the tabs when the selection is changed. However, even in this case you will need to select the tabs at least once in order for its contents to be loaded.
As a side note, I just want to note you that there were a duplicate of this forum post and I removed it.
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