Thank you for the sample project. I see the problem (without even running the code). Your common.cs must not use BaseWebAiiTest. This base class only gets properly initialized when its the code behind file for a test.
The problem you're running into is that the Wait object hasn't been initialized when you execute:
Wait.For<RadGridView>(rgv => rgv.HeaderRow.HeaderCells.Count > 1, grid, 15000);
Instead you could pass in "this" as a parameter to the constructor for Common and use that throughout your methods. You should also eliminate the Pages property in your Common class. It does not belong there.