Something in your code is resolving to a "null" and you are trying to access a property on it. We can't tell you what that is as we can't run your code, you'll need to use the debugger to find out what. It could be "Cadapter", or Session["CCats"], or Session["CQuali"] and so on.
Once you've found out what is null then how you solve it depends on what you want to do in that situation, which again we can't know, but it's going to involve code like
if (XYX == null}
// take the appropriate action
This question is asked multiple times a day, google the error for many articles discussing this, please do your own basic research before asking a question.