May need some more info:
- What version of Kentico is this
- Did you modify any of the Kentico core admin pages/files that may be a cause of this?
If you need a quick work around to 'fix' the situation, i would make a copy of the original PreviewHierarchy.ascx, then modify the original and replace
<%= PaneContent.ClientID %> with
<%= (PaneContent != null ? PaneContent.ClientID : "") %>