I did that a couple of times, but it seems that everytime the solution is rebuilt, I get a different error - and still am not able to look at MainForm's design. For example, after the first rebuilt, I get the error:
Could not load file or assembly 'Telerik.WinControls.UI.Design, Version=2009.3.9.1111, Culture=neutral, PublicKeyToken=5bb2a467cbec794e' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
After the second rebuilt, I get the same error, plus:
The variable 'treeViewDemoNavigation' is either undeclared or was never assigned.
I even restarted VS after the rebuilt, but the error still shows. I check my GAC and found that for Telerik.Wincontrols.UI.Design version 2009.3.9.1111, the PublicKeyToken is NOT 5bb2a467cbec794e. Instead, it is bf4391287131aaeb. Instead, version 2009.3.9.1103, which is also in my GAC, has the PublicKeyToken of 5bb2a467cbec794e. So I'm not sure what's going on there. . .
Thanks again for your hlep!