This error first appear to occur randomly.
Steps to recreate:
So the problem appears reproducible.
However, when trying to narrow it down to a single file (that is automatically opened when the solution is loaded), I couldn't reproduce the problem. Now with all the files open again the problem doesn't occur!!! So it looks like it is fixed - though this happened before and eventually the issue came back.
I think it is to do with one of the user controls with DevExpress controls on it - when the error occurs, the designer displays the error. Though I can't reproduce it at the moment to confirm that.
This has stopped happening for the time being, so I can't reproduce it.
Try to catch the exception and post the callstack here. To grab the callstack, try to use the approach described in the What information do you need to fix a problem, which occurs with your products at design time?. Also, please make certain that the CopyLocal attribute for DevExpress assemblies referenced by your project is set to false. Also, the bin folder of your application should not contain our assemblies. Perhaps, this will help you to address this issue.