Unfortunately I could not replicate the error within your example, which makes sense because I am doing things a bit differently on my end. In my application, I am using a DBML as my Datasource which is queried by a Linq statement within the "OnNeedDataSource" method on my RadGrid.
Fortunately, I was able to find out exactly what is throwing the "Object reference not set to an instance of an object" error. The error is being bubbled up directly from my DBML's Designer layer ("datasource.designer.cs").
One of the fields that is throwing this error is my "Account" field on my view. When filtering on this field, the error is thrown immediately after the RadFilter loops onto the first record with a NULL as Account. I've shown the Designer code below.
._Account; //THROWING ERROR WHEN THIS FIELD IS FOUND TO BE NULL
._Account != value))
._Account = value;
NOTE: I am mainly attempting to use the "contains" expression type, but the "IsNull" expression type seems to work perfectly fine, and finds the NULLS.
I don't believe it's my DBML that is the problem because this View Binds to all of my Grids fine when not being searched (with and without nulls).
I noticed in another thread that one of the Telerik Reps asked "Does your database contain Nulls? Try removing them and then testing" but no one responded. Is there a potential problem with the RadFilter and Nulls in the Database, in some situations?