Rotating a bulge around a null object

You are over-complicating this and are forgetting/ ignoring that effects operate relative to the layer, not the comp. the coordinates from your rotating glass would need to be converted using layer space transforms such as toComp(). The circle can easily be created using the effect of same name instead of a mask. Simply set the rendering mode in the effect to Stencil Alpha. Avoids another step off coordinate conversions. Other than that you simply may want to reconsider your setup. If you really want to "rotate" your magnifier glass around an imaginary center copy & pasting a suitably sized mask path top the position property after you created it on a matching comp-sized solid might have been a more beginner-friendly approach rather than putting up with this rotation stuff. Then you wouldn't need extra math to counter the rotation and your simple pickwhip stuff would work right away. Either way, it might be a good idea to read the online help and find some good tutorials on some of these basics.

Mylenium

I am not sure that I completely understand your workflow. I think you want to rotate the magnifying glass graphic, the null that is showing is not in the center of the magnifying glass, but I think that is the center of rotation.

To attach the Center of the Bulge effect to the Center of the Magnifying glass you have to move the anchor point of the Magnifying Glass layer to the center of the lens. It will still rotate around the null.

Then you have to use layer space transformations in an expression to get the comp position of the center of the magnifying glass layer and apply it to the Bulge effect. If you had included a screenshot that shows us the modified properties (press 'uu') of your layers I would know for sure. This is what that would look like, and I would probably use Magnify instead of Bulge, but here goes:

Here is the expression:

L = thisComp.layer("Magnifying Glass");
L.toComp(anchorPoint)

I even uploaded an AEP file for you to look at.