Invalid Resx file. Type in the data at line 137, position 5, cannot be loaded because it threw the following exception during construction: Only one DBNull instance may exist, and calls to DBNull deserialization methods are not allowed

Only in Visual Studio 2012 do I get the DBnull error and it occurs if I make any change whatsoever to my Windows form.

I can still edit it in Visual Studio 2010, without error.

There are two calendar control on the form, both of which allow null as the date values may not be know by the user. 

Also, loading this form in Visual Studio 2012 takes about 9 seconds whereas under VS2010 it takes less than 1 second.

Can you suggest a resolution to this?