Walk 'object tree' with easy null handling

Looks good!
 
I would suggest using Func<T> instead of GetDelegate<T>, simply because it's the same thing.
 
Also, the compiler will deduce the type for you, so there's no need to write:

Level3 level3 = NullHelper.Get<Level3>(() => level1.Level2.Level3);

when:

var level3 = NullHelper.Get(() => level1.Level2.Level3);

may be easier to read.