Remove null warnings

Eclipse's null analysis thinks that a couple of references (required
to be non null) might be null. We can tell it we know better with an
assertion.

That's because its analysis doesn't go deep enough to for example
understand this:

     Object x = null;
     boolean y = true;
     if (y) {
        x = "not null";
     }

     if (y) {
         // Here we know x is not null, but Eclipse does not
     }

Change-Id: If63db13feabf05832c5d9e488ace5c72224da554