I have tested this on two of my sites that were upgraded to 4.3.6. When I set the Host Settings > Advanced > Other > Control Panel from ICONBAr to Classic I get the following exception.

An error has occurred.
DotNetNuke.Services.Exceptions.ModuleLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.UI.ControlPanels.Classic.Page_Load(Object sender, EventArgs e) --- End of inner exception stack trace ---

Is the a bug or a feature caused by upgrading?  I am able to reset this value and continue.

This is not a new bug - I suppose it is one, but it's avoidable, simply by removing the control panel elements in the skin and letting DNN handle it automatically  - but .. when you have the controlpanel code in the skin file and you use the classic one, comes up with that error becuase it's using a reference that can't be found.

Try deleting any control pane references in the ascx file - since I'm assuming that's causing the error.

I could be wrong but this is another reason I don't put the control panel elements into the skin, I put it into the CSS file, but allow DNN to place full width across the screen.

I hope this helps.

Nina Meiers