We have same problem. Some Form/UserControl classes can not be viewed in designer and Visual Studio causes various exceptions.
There are one typical cause:
One of designed component thrown unhandled exception during initialization ( in constructor or in Load event or before ).
Not only for this case, you can run another instance of visual studio, open/create some independent project, go to menu -> Debug -> Attach to process … -> select instance of devenv.exe process with problematic designer. Then press Ctrl+Alt+E, the “Exceptions” windows should be shown. There check “Thrown” in categories of exception.
Now active the visual studio with designer and try view designer. If the exception will be thrown, you will see callstack ( and maybe source code, if the exception was thrown from your code ) and other typical information about thrown exception. This information may be very helpful.
If you have something like TypeLoadException from Winforms designer, when debugging Visual Studio (devenv.exe process) with another instance of Visual Studio, have a look at the Debug > Modules panel to see exactly which version of your DLL is loaded. Turned out that it was an unexpected version for us, hence the issue.